My best friends sis is a lesbian...?

Its not that I don't like lesbians its just that my best friend has a 17 yr old sis.(BTW I'm a 13 yr old girl.) Anyway 4 weeks ago I went round her house and we were sitting there playing dare, then she went toilet. While she was gone her sis came in only in bra and a thong. She looked at me wiv a smile and then took off her bra! She then came over to me and befor I could push her away, she kissed me! I was like wtf just happened!? Anyway now Wen I go round my best friends house, I try and avoid me bein on my own in a room. but Wen I am she's been doin it again. yesterday she went way way way to far. she came up to me naked!...and started pulling my panties down from under my skirt! I was struggling and I was hitting her saying get away from me and so on. she then put her finger in my hole and finger ****** me! Then she started licking me!!! But the bad thing was I enjoyed it. She brought me to orgasm... She then told me she was a lesbian and that she loved me. But now I'm confused because I'm not a lesbian, and I don't want to be. I feel uncomfortable around her sister, as if she's trying to rape me. But the worst thing is, my best friend doesn't know. Wat should I do??? I've already confronted her 17 yr old sis but she won't listen.