Yeah, first of all let me tell you, Japanese authors usually write interesting stories but they suck at ending them.
The Marie Route is fantastic at first but it will become utter sh*t in the third act so here are the spoilers for the last two chapters (which were a special bonus for those who bought the last volume of Mobuseka in Japan only):
So in the last two chapters Marie and Angelica reached the throne room of the castle where Anne Olivia (Olivia being possessed by Anne's spirit that was in the saint artifacts) is facing Leon.
Something that's disappointing is the retcon the author does with the entire plot and characters in the last two chapters. It's like she thought "wait a minute, how come Marie is getting a happy end?" "Hell no, she has to suffer and this isn't harem so it's no good".
So Leon went back to being a hesitant coward little sh*t. He clearly knows Olivia needs to be stopped since a lot of people are dying because of her. The king and Leon's master are dead (they couldn't stop Olivia and she killed them both in a fight). Yet Leon doesn't want to fight or kill Olivia. Let me sincere here, nobody really gives a f*ck about Olivia in this route, the only reason she remains relevant is the Author being a d*mb b*tch obsessed with Olivia. She can't let go of this st*pid character.
Olivia is blackmailing Leon, saying she will kill herself unless Leon becomes her man before Marie appeared. So Marie has to do what Leon can't. She faces Leon and tells him "he should deny other girl's approaches because he is already engaged". Imagine being a male character so f*cking MID needing to have the girl tell him how a real man should act... She forcefully awakens the powers of the Saint within her (it was revealed in case you forgot that Marie and Olivia are both decendants of the first saint Anne. Yes this is the very same Anne currently possessing Olivia. Also to clarify Olivia allowed Anne to possess her because she was tired of people reads the papers not letting her study for the school exams... okay)
Anyway Jilk appears in a mecha to defend Anne Olivia (Yeah Olivia is called Anne Olivia now), Anne Olivia kills him from behind like a mf. Julius appears to defend Anne Olivia, Anne Olivia pretty much leaves him at death's door from behind. Angelica (who is madly in love with Julius even though he basically abandoned her) goes mad and tries to kill Olivia but gets pushed back by her power. Then Marie beats the sh*t out of Olivia (cause Leon is a pos who suddenly became the Leon of Mobuseka that was a lazy coward). Anne's other spirit comes out (cause the first saint Anne divided her spirit in two possessing the saint relics or artifacts with both halves of her spirit) and talks no jutsu the evil Anne possessing Olivia into calming down.
The thing is Anne was in love with Leah (a hero she met in the past along with the founders of the kingdom) who happens to be Leon's ancestor. So the "good" Anne talks the evil Anne into going both of them inside Marie (who is their decendant) because she is in love with Leon and they will get married, evil Anne accepts and everybody happy...
Or so that would have been the more or less good ending but author said, "nah, let's make it worse). So Anne's sister's spirit Mary came out of the saint Artifact, sealed both Annes onto herself and took their power. Marie is fed up and Mary forces them to watch a longass flashback where it's revealed Mary met Leah and fell in love with him first but she was too shy and not as sociable/popular/beautiful as her older sister Anne who blatantly fell in love with Leah. However the other people who founded the kingdom (that is Julius' and the other 4 d*mbasses ancestors) were jealous of Leah and made him fall into a trap where he supposedly died. This made Anne and Mary fall in despair and hate the kingdom promising to have their revenge. However Mary was even angrier so she sweetalked Anne into sealing her soul in the saint artifacts with the hopes to wake up in a world where Leah would be alive. But in fact it was a trap to make Anne suffer for getting between her (Mary) and her beloved Leah back when "he was alive". Sealing Anne's soul while her (still breathing body) would be sold to some rich aristocrats that will r*pe and humilliate her. Anne didn't know the whole time.
So back after the flashback Marie and the others fight Mary again and barely beat her. Everybody think it's over but the alliance of nobles are still fighting outside the royal forces of Olivia.
Then suddenly Leon is shot by somebody in front of Marie who falls in despair trying to heal him with the last of her powers. Marie looks around but everybody pretends they are innocent. Then Luxion reveals himself, claiming he did it because Leon is no good by being friendly to new humans who must be eliminated. He lost the contract with Leon thanks to (accidentally) fighting Anne Olivia days prior. He takes everything and flies away into his main body (spaceship) to keep annihilating the new humans.
Marie begs Olivia to help her heal Leon who's with one foot in the grave because she almost has no magic left. However before she could do anything, Arroganz appears (maybe you forgot but Arroganz has a new AI Luxion left before going away on his own around halfway in the story). Marie covers the dying Leon with her body ready to be killed by Arroganz (both Leon and Marie basically raised this AI like their own child, yes Arroganz is also the name of this new AI and it started with the mindset of a kid but it grew up as it was learning more). However this Arroganz wasn't under Luxion's control and took everybody to Partner (Leon's ship), healed him and went into hiding.
Meanwhile Luxion with an army of AI piloting robots went to massacre everybody friend or foe. Even Nicks (Leon's brother who was actually married to Dorothea in this timeline) is burned to death by Luxion's ship heat ray along with his entire fleet (its not shown but Leon's poor noble friends were also fighting so I suppose Luxion also killed them off screen.
So the last part (and worse of all). Author said "how should I sh*t even more on this story... oh I know, let's retcon Leon and Marie's relationship and kill one of them off".
Leon wakes up, he was saved with the effort of everybody. Instead of cherishing that life Marie and everybody lost so much to protect he decides to go fight a lost fight like a mf. But before that he goes with Marie who's worried sick about him. He talks a lot of crap to confuse her and then tells her "it's like that time when I saved you from those guys trying to pick you up" (or some sh*t like that), Marie says "yeah you were so cool back then..." then she realizes she fell in the trap (that was an anecdote from their past lives).
Then Leon becomes an *sshole. Marie starts crying and he says "Oh I was about to kiss my sister, sorry let's cancel the engagement". He goes to Arroganz to board it then Marie tells him not to go but he says nah "I'll be back exclusively to cancel the engagement". She says "you will never ever find a better woman than me" he says "yeah" and takes off. Marie falls to her knees crying.
Leon goes to one of the least anticlimactic fights ever defeating many AI robots and gets to Luxion.
Luxion confesses he actually didn't want to hurt him or Marie but ends up killing Leon. Before he dies Leon says he "wishes Marie is happy in the end" (mothrf*cker is this a joke? After breaking her heart and basically turning her into a widow that's the first thing he says? Also the author laughing at everybody here I bet).
So Luxion is also fatally wounded. He says Arroganz to go protect Marie (how many more hypocrites are here?)
Arroganz returns in front of an expectant Marie who (surprise surprise) finds its cockpit empty. Arroganz tells her Leon died and his body exploded with Luxion's main body (spaceship). Everybody (who's still alive) goes to celebrate while Marie falls on her hands and knees crying her lungs out.
All in all everybody died except Olivia who survived with s*icidal thoughts out of guilt, Angelica who's traumatized by prince Julius being murdered in front of her and Marie who's heart was broken into million pieces and tossed into the sun looking worse than ever.
She boards Arroganz and wishes "if there is a god all she wants is to repeat this story and meet Leon again, then she would make their happy ending work this time" then leaves.
end.
postcredits scene: Leon appears followes by Luxion. He goes to the school garden where prince Julius is then he sees a petite blonde girl with sky blue eyes. She stares back and asks "what?". Leon says "Nothing" then walks past each other. She goes to where Julius is and he walks towards where he spotted a new girl, Olivia (yeah the author won't let go of this d*mb b*tch like she is some well written female character. What a joke)...
Like I've said before if you will write a route where you will kill everybody with the justification of giving Marie a happy ending at the very least give Marie the f*cking happy ending... otherwise it will be a waste of time...
Also no they aren't siblings anymore. This is some bs. Author got cold feet and bent the knee to puritans by retconning the whole thing at the last minute.
Now about Olivia and Angelica in the main route (Mubuseka) yes they cucked Leon. Confessed to each other then kissed and had s*x. Leon found them, hid and saw the whole thing. Then decided to take the L and leave.
Many things happened after with Leon distancing himself from them not to be a nuisance.
But author said "wait a minute this will make my dear Olivia less relevant in the story gotta make her a harem member somehow".
So Olivia and Angelica decided to confess everything to mr. Cuck-eon then he admitted he didn't want to be a third wheel but they said "let's be your girlfriends but keep our lesbian relationship" he said "okay". So technically they are part of the harem but also f*ck at Leon's back like a good japanese cuck he is.🥴
So yeah it's up to you if you wanna keep reading.
Personally I recommend a better novel called "Ending Maker". It's what the Marie route PRETENDED to be:
Two earthlings (boy and girl) reincarnated in a game, gotta work together to break the game scenarios, seek to achieve the happy ending for everybody, use their game knowledge and help each other, travel together all the time, won't allow others to interfere with or get in between them.
Great fights and the interactions/dialogues between characters is 10/10. Romance at great pacing, no f*cking harem, it's finished and even has bonus chapters after the main story, great world building, lots of phenomenal plot twists and its all in english. All of it. Hope all this helped a bit.
So a few things i gathered from that:
1. Author has brain damage. And writes HARD-NTR, with leon becoming a third wheel cuck, while the lesbian pretend he isn't, but in reality he is. (and there is zero harem, which brings me to point 2).
2. Author never wrote in any route ANY harem, and this is only poly/sharing bullshit. (for those that don't know the difference, harem is where the woman are EXCLUSIVE to the man, i.e. no lesbian relationship shit, at MOST there can be a bit of touching in MFF, but you pull out toys and strap-on then it's sharing/poly, and zero harem).
3. Marie route ends with "random bullshit go" with author yet again doing NTR, but to the fans this time.
4. while you never specificially mentioned it, Anoseka is basically marie route and will end just as shit? (this is a statement, the question is whether or not this statement is correct).
5. I should drop this shit, harder than the author was dropped when they were a baby. (I use sexless terms, cause while you said she? I ain't sure of the authors sex.)
6. I should go catchup on ending maker... which I can confirm is a good read, I just lost my place.
7. Assuming the author is a woman... I have yet to see a female writer actually understand what harem is, and write it well, without some lesbian shit or NTR. (where the man goes cuck and takes them back... don't be like forest gump people you will get aids XD ) the one tiny win, is that they didn't take notes from the after story that mushoku tensei did with the sister (kidnaps the son and grooms him until 11 to do the deed... yup, it went over as well as you'd think, and author still plans to do it... is why I'm never touching that trash ever, author needs to see a shrink cause they have issues).
8. I think I covered everything at this point? I didn't use spoilers cause this is info worth spoiling to save people from that dumpster fire.
I leave you with but one question... is there any chance, in your opinion, that mobseka or more importantly Anoseka, won't follow the trash that is the novels? (i.e. they won't actually NTR MC or do lesbian bullshit and call it harem in mobseka, and anoseka will actually have what was promised in marie route.. i.e. they end up together).
I remember when authors would actually commit to the premise... like Oreimo (never read or watched it personally, I just now there is at least one ending where they end up together).
Now they all do random bullshit go in the last ark and cuck out. (like making them not actually related with sudden retcons... Oreimo to my knowledge only did that in the games).
Too much western influence on japan... removing of wholesome Alabama (and even normal relationships), for some weird cuck fantasy fetish.
Yes, I know NTR as a term comes from japan, but it's a fact west popularised the shit out of it, like japan has laws where you can sue your partner for cheating and most towns will shun them for it... just probably not tokyo or kyoto.