A couple of reasons:
1) He is her brother
2) She's a virgin
3) There is attraction but also fear, on both sides.
For a lot of people, the step between fingering and fucking is rather big. As are the steps between kissing, seeing someone naked and fingering.
I think me and my girlfriend spent about a year to go from kissing to fingering and then another year to get to fucking.
And no, she is not my sister.
1) Not technically, can't be on Patreon, so the argument is moot
2) And so are half the chicks in these games that immediately decide to pogo on the nearest dick. Even when they've got kids some how. This is done purely for the kink bc she at no point behaves as a virgin
3) That's EVERY first time when there is an emotional connection
There is clearly a massive difference between what you experience in your culture and what is actually common in most of Western society. We live in a world where you have girls in HS opening OF pages. It's been nearly 60yrs since the "sexual revolution" and we've reached this crazy point where half the country is whoring themselves out to the lowest bidders while shaming men for looking at anything that resembles a human female. I was going to say the only virgins you'd find would be on some cults compound, but even that's a lie bc they're marrying their kids off to men 4x their age who already have 3 wives. So this entire kink is like lusting after bigfoot. And, at the end of the day, Chloe is waaaay too big of a slut for it to be anything more than lip service. That's not a complaint, it's just laughable how frequently she puts herself into a VERY sexually charged situation with clear intent.
The Grace, MC, and Chloe relationship just doesn't make sense in any context. You could argue Grace is the MC's much older sister, Chloe her granddaughter, MC Chloe's great uncle. These age gaps are just wild and illogical for any scenario. I think the dev just realized that there is no fixing it w/o pissing people off, so he's just ignoring it and frustrating everyone.
Once you ignore the virgin bs, the characters ages and the "relationship" between those 3, it's an interesting story. Those are relatively small hiccups for most AVN's on this site. And it's far from the only game who slaps specific kink tags onto their game and then barely gives them the lip service necessary to justify them. It'd be easy enough to fix and make sense of it all, but I think we'd all agree we'd prefer they just keep moving the story forward. Once it's done, then they can come back and fix this shit.