Search Results: cheating friend with benefit

OK I met this girl at a Halloween party a couple of weeks ago. We've been out together a lot in that time frame, like 10 or 11 times. This is a serious relationship, by the way, she's not just a friend with benefits or a f*** buddy. We're both really enjoying each other and there haven't been any awkward moments or anything. We've never had any arguments or gotten upset at each other. Basically, it's going really well.

The Bad News. She has a really hot sister who is a total slut. I don't think there's anything wrong with that, but I'm not looking for sex right now. Anyways, her sister knows me because she is a friend of a girl I was with a while ago. She knows that I was a player in the past and she tried to seduce me. I told her: "I'm serious with your sister I'm not having sex with you." The next day she told me that if I don't have sex with her soon, she's going to tell her sister that I'm a cheating womanizer and that I tried to have sex with her. I'm thinking that if I do have sex with her, she will just tell her sister anyways, because she's a total bitch. She's a girl version of a player. And if I don't have sex with her, she's going to lie to her sister and compromise my entire relationship.

Advice? I really don't know how to handle a situation like this and I don't have much time before her sister will approach me again.