After the MC confronts her after the barn scene happens he threatens her with blackmail and she basically offers to be friends with benefits if he doesn't grass on her to your Mother.
Yeah still not the girl from your dreams and then to do it as dev with one of the two most wanted girls, no idea why devs think it makes sense even in a incest situation that would anyone want to end up in a relation with her beyond just using her........ There are better reasons why someone would end up in an incestuous relation especially in a situation there are not 100's of other men around. -shakes head- Just like also for mom and sis have same mechanic and starting at same point as all other women.
Well clearly a loving family or complete asshole MC, dev might even have been better off not including incest at all then do it in this way.
Edit: Nor does the turn around from your sis makes any sense, unless the only thing she cares for is money, well what a good reason to become the long term partner of you sis when all she cares about is how much money you give her ........ Really the girl of my dreams.