What do I do?

all right I have a friend (Rachel) last year she was my best friend, so I became her boyfriend so it lasted but this year we broke up. I realy wanted to stay friends with her by puting aside what happened. she agrees and so some time pes and were friends but then I went to a party that my friend did. there I ran into Rachel's sister and we start talking and we realy hit it off ( we didn't kiss or anything like that).after that we become realy close then I started to like her and then wensday she tells me that she likes me (she still dosen't know that I like her). now I don't know if I should tell her or not because i'm afraid that I could mess up my friendship with Rachel. what do I do??? help!!!!!!!!!!!!!!