I know this probably wont be super helpful. But the main thing I remember about the game is the sister wants a job or needs money so she works at like a kinda brothel. You get a mask and become her first customer and I remember like teaching her to swim and stuff. You get the option to convince her to stop working there. I play too many of these games and they all kind of blend together but I remember enjoying the little interactions in that side quest.
Honestly this game might be what I was thinking about. I kept overlooking it cause I remember other things from this game not that plot but I'll check it out again thanks!