real story: Author HATES little sister characters which is why being Marie is suffering. See his other works for more examples.
but she turned out to be stupidly popular anyway, so she's here to stay as a result, including getting her own spinoff.
as for Leon, he's not in a game and he knows it. His avoiding Angie and Olivia up til now was a combination of him not thinking he was good enough and that marrying either of them is antithetical to his goals of just living a quiet life in the country without any of the drama or bullshit. Marrying into a Ducal house or to the protagonist who gets drawn into stuff is the exact opposite.
Also he wanted to screw around with his newfound Hero status for a while. getting him engaged immediately was the Kings way of screwing him over in that department. The Manga left out a few details in that letter.
Leon has spent a lot of time rationalizing his actions in a way that indicates the opposite of what you said. He is aware that he is in a world based on a game he played in his original world, yes--but he has
long treated it as if it will follow that game's storyline exactly, and he has explicitly named something to that effect as his reason for not pursuing Angie and Olivia very early on. He completely ignored all the organic differences in events to tell himself that
he has to avoid changing them. (In addition, remember: not only did he insistently and constantly brand himself a mere "mob" early on,
it's in the name of this story.) Over time, he began to make other excuses--to the point of shifting between them in short periods of time.
As for the author's "real story", that's a tough sell. He hates little sister characters, you say. Indeed--he hates them enough to write an entire spin-off LN about one. You can call her Marie's circumstances "suffering"--certainly, she's experiencing unpleasantries, even as a result of her actions. The catch is that she never gets her just deserts for her deeds--ever--and that is by the pen of this author. She will screech and weep conspicuous croc tears about her circumstances--bad circumstances that will keep coming for however long, until she gets the inevitable "redemption" arc--but nothing that happens to her is ever enough to stop her from doing the things she does, no matter how flagrant.
I've seen what it looks like when an author is in opposition to or otherwise bears ill will toward one of his own characters. Sure, the author may have the character suffer like anyone else--just as Marie does here--but that character's suffering will
not be superficial as it is for Marie, and that character's depiction will
not be positive. Receiving positive societal/social regard--even if a source of annoyance or foolishly rejected--is positive depiction. Constantly being saved from one's own foolishness or vicious behavior, even if it merits death, is positive depiction. A character receiving not only a major role in a story that isn't about her but also her own spinoff focused on her is positive depiction. So on, and so forth. Of course, you can tell that all three of the aforementioned apply to Marie. Marie has more than just plot armor, and despised characters don't get what she does.
Besides, Marie is indeed Leon's little sister--that's a fact--but that's not her
only character trait, nor is it her primary character trait. Her
actual primary character trait is the reason the author loves her so much.