So... they both thought they were blood related, one of them felt that was a deal breaker and tried to peace out of the weird relationship, the other was weirdly into it. It turns out they aren't related, but the one with an understandable dislike of incest still doesn't want to deal with this weird yandere who she was raised alongside, and the yandere is trying to double-down with the "see we're not actually related by blood, we were just raised together like siblings, that totally means we can fuck guilt-free".
I think that's everything?