My Sister

I'm 14 and my twin is my best friend. One day I was sleeping outside and she came to join me and we fell asleep and then I woke up and she was full blown nude. I just ignored it not caring but then when I went to go get water and come back and I see her master baiting with NY stuff and she sees me and freaks out so as I go to sleep inside she started to touch me. I freaked out and told her to stop but as I was she kissed me and then we ended up making love for almost the entire night I want to stop but she won't leave me alone what should I do?