don't be. it will be a huge turn-off for a lot of potential partners. dating a girl like that, in such an incestuous friend group, why bother? you're just gonna feel uneasy whenever she hangs out with her friends, as they also have a sexual history between them. and once you've crossed that line, obviously it will happen again more easily a 2nd time. it's kinda like dating a girl who is still close friends with her ex - something most people wouldn't be interested in. and also, if she did cheat, since that friend group is so incestuous, you would never hear about it. you may be one of their romantic partners, but you wouldn't ever really get in between them. their friend group would always have top priority over you. so if you get cheated on by people from that group, they wouldn't care.