Some games are good, and even excellent, but I really don't understand this trend of incest stories and even more with very young MCs. Is it really problematic if the MC lives next to a house with a lot of women, or is a foreign student for exemple, rather than the little brother ?! Honestly, I don't understand.
In my opinion it comes down to a couple things. I feel like it started becoming big in this industry because:
1) A sexual attraction to ones mother is a well documented phase in puberty (Oedipus complex). I would argue that most hetero males who went through puberty while living with an attractive mother and/or sister, atleast thought about what it would be like at some point. Shit, I remember being curious about a much larger variety of women while I was in puberty than I would ever consider as an adult. Anyway, I think these games introduce situations similar to some of those puberty influenced thoughts. So the similarity makes it a little nostalgic and a bit more exciting than it otherwise would be.
2) There is a lack of these story lines in mainstream porn. Milf porn is entirely separate, at most you will see a stepmom (usually much closer in age than a biological mother could be) and thus not incest. You really have to go to either Japanese manga and games to see common incest themes. There is a smaller company that does some incest porn called RedMilf productions (I think?), but again not mainstream and not easily accessible. So these games filled a mostly empty niche for english speakers.
But really, at this point, you are seeing so many of them simply because of the success of the early adapters. Icstor ( Milf Control and Incest Story) was incredibly successful with his games on patreon early on. Add to that Big Brother and Dreams of Desire, and probably some others I'm forgetting, and you have a whole lot of incest success stories for newer developers to emulate. You can almost tell time in successful games in this industry, each really successful game seems to start it's own new era. Incest Story started a mother/sister incest era, then Dating My Daughter came along and started a father/daughter incest era, and most recently My Sweet Neighbors has started a trend away from incest and toward neighbors.