To be honest, I just played this and had the intention to comment on the fact that it's probably the quickest introduction to "forbidden" sex I've seen in any game I've played that doesn't feature an already established couple.
That being said, this forum has thousands of different games, each with their own approach at sex, but a ton of them will take you on some kind of journey to give each relationship a somewhat plausible foundation. This is clearly not the case here. This is for the player who wants to see incestual sex between two consenting teens (and thin, pointy tongues) as quickly as possible. Do you want a reason? Hormones, most likely. Loads of them. I don't know about you but many years ago, when I was in high school, some days I was so fucking horny I literally couldn't think straight. In those days, I would've fucked ANYTHING given the chance and if I'd had a hot sister at my door asking to suck my dick, I would've done it and then, maybe, regretted it post-nut.
So, between this and the classic story where a widow/divorced mother feels lonely and starts talking about her handsome son and how much he's grown in the past few years, I'd say this is way more realistic.