What should I do about my GF/fiances's niece who constantly flirts/teases and tries to seduce me?

First of all we aren't that far apart in age...I'm 25 and she is 18. I just met her over the summer and she is an insatiable flirt/tease. When we visited last summer she invited me over to baby sit with her (my GF thought it was a great idea), when I got there she was dressed in the tiniest pair of shorts and a tiny halter/belly shirt (the kids were sleeping btw)...I admit she looked brutally hot, she kept teasing/flirting and eventually ended up in my lap. From there nature took over and we ended up in a really heated make out session. I was really tempted to drag her in the bedroom but for several reasons thought better of it. I left shortly thereafter and when she dropped by the house after her baby sitting she was dressed in jeans and a tee shirt...so she had obviously changed clothes at the baby sitteres house after she called and invited me over. So she planned what happened is what I realized. I admit she is smoking hot and being a young guy I am very physically attracted to her. That and her being a little vixen makes it tough for me to be around her. Anyway we are going back to visit the in-laws for christmas so I will be like 3 houses away from her for 2 weeks. I guess I am wondering what to do. I haven't mentioned it to my GF (she is almost my fiance). Being honest with myself I am not 100% sure that I won't end up with some "alone" time with the tart and I know part of me wants to, but I am trying to resist the temptation....help???