How to get a girl to like u even if she likes someone else.?

Okay, here's the story. So I went on vacation with my family. I brought a friends and my sis did too. After a while I really got to know my sisters friend. We stayed up all night on the deck and talked, took a long walk on the beach together and sat out in a thunderstorm together. I didn't make any moves on her but I really like her. Only problem is that I overheard her and my sister talking about a guy she likes. We r back from vacation and I asked her if she liked the guy but she said she doesn't know yet. I have no idea what to do or how to say it and I'm afraid that if I don't do something quick I won't be able to get her. Help!!!