Wondering; does she like me?

About three weeks ago I met a close friend's sister. Let's call her C. So I've been hanging out with her and talking to her as much as possible, and all the signs are there. She laughs at everything I say, she twirls her hair when we talk, etc. But- she has a boyfriend. All of her friends keep telling me that she seems to like me, but I'm not quite so sure. If I ask her, I might ruin a friendship that could've been awesome, but if I don't, I might never have a chance to be with her. Should I tell her that I like her a lot, or should I try and cope with "just friends?" Thank you for reading.