Honestly I'm glad that MC's mother is only plot character. There is already dozens of games, where mothers only represent as sexual objects and not plot drive characters, it's diffidently a good thing that this game went in different direction. Personally, I'm fine with incest in the games, but I'm strongly against incest toward moms (I don't against those who like it), even despite that I don't self insert myself in hide of protagonist, the idea of having sex with person who gave birth this person make me uneasy, to say the least. But, for some reason, I find it hot to bang sisters in the games

Maybe because I was was the only child in the family, so I just didn't have sibling relationships. Also as I heard from the others who has siblings, they usually not in that good relationships towards each other. In some stories they are almost enemies, at least in teenager period. But after they get mature and have their own families, the situation is getting better. So it's pretty fun for me to read this stories about pure sibling love. I don't say it's never happens, but keeping in mind how much weird this protagonists are, it doesn't look natural at all.