[adpiucf]

Just because someone shares membership in the same organization as you does not make them your instant best friend. A sorority is there to make college a better social and academic experience. You don't have to be BFF with every person, and if someone starts seeing a guy who you had history with, you need to deal with it in a way that suits you without causing a catfight. Unless this guy is worth a catfight. Your best friend wouldn't do that to you, but this person clearly isn't your friend. You're still going to have to suck it up and be at the same functions together, so better to deal with it now like a mature adult and talk with her. Girls have this great habit of not telling each other how they're feeling, but love to spread the word to every other possible person who will listen. Confront her, yell at her, but deal with the problem. And if she really is causing the chapter so much trouble in so many ways with her promiscuity, as you have alluded, that is a chapter standards issue, because it deals with chapter reputation, not to mention that woman's physical and mental health risks. I would hope a standards committee would talk to her about her actions, both for her own good before she "catches" something or gets hurt, and before the Greek Grape Vine starts lableing your chapter unfairly due to the actions of one member.