So far Alex didn't do much crazy stuff she even told so, but with Ann working on her and MC maybe working on her, it would be possible happening, specialy if take path where Ann go college then MC got more time to spend with Alex.
Yeah, Alex may be less promiscuous than Ann for now, but she already dresses sluttier/more liberally and seems pretty confident most of the time. Which makes me believe she will be willing to get wilder than Ann, in time of course.
Emily on the other hand, not sure if she would lower herself as much as Ann, she is older, got some experience and by the look so far she rather would go for one guy, maybe even share her guy with others if the guy is the MC.
I just disagree with this wholeheartedly, she was so willing to have casual sex with her ex when he came over. And It's not about lowering herself as far as any other character. It's about being willing to explore a lot of different things with her brother, and I wouldn't be surprised if it was in the sharing/exhibitionism realm, and who knows what else.