Reading the comments, it's not only the mana that gives the male twin a disadvantage of sorts. It's like, there's not a single decent young or young looking female character (empress/queen where?) that isn't mean (other than protag-chan) while there's ton of decent young male characters, not saying that they need opposite gender people around it just shows the preferences of the author and for a manga that is committed to the plot (I'll assume because I haven't found anything funny), showing too much of those preferences is something that irks me.
Plus most if not all of the perspective is of the female side and honestly I don't think the plot would change much if he didn't actually exist like I'm sorry but you added twins but that honestly doesn't impact the plot much since they have similar reactions to everything.
Also in medieval times twins were actually a sign of bad luck or the devil, (because conjoined twins looked like hellspawns to them so they don't like normal ones either) they didn't actually need the twins to be illegitimate if they wanted the twins to gain the pity or despise of people around them and it would've actually contributed to the plot that there was two of them cause let's be real, the fact that it hasn't impacted anything despite being a selling point is just off-putting.
I'm sorry for being so negative about this despite having gone so far into the story, so much so that people would wonder why I'm still reading it since this is only one of my many complaints that I have on this webtoon. (I don't want to list it all because I usually get annoyed myself when reading negative reviews lat-ish into the manga so writing too much is hypocritical to say the least.)