I think if you tell about her, the focus would be on you rather than on her. It would seem like sour grapes and no one would believe you. It sounds like she has her own sexual addiction, and people like that usually shoot themselves in the foot sooner or later. When it happens, then you can tell your sister, "she wasn't as good as she pretended to be. Maybe I'm not as bad as you think I am." You can talk to her a little about the dangers of judging others.
so my opinion is that you should enjoy your revenge fantasies just as much as you want, it's fun to think about what you'd like to do to smarmy bitches like that, but don't actually carry any of your fantasies out as I think it will come back to bite you.
mind your own business, let the woman work out her own karma, don't soil your hands digging into her mess.