I think I like her.......How do I tell her?

I think Im starting to like my best friends little sister. She is cut, athletic, korky in her own little, smart, funny i could go on but i digress. We haven't hung out just the two of us ever when ever we talked or were togeather her brother was there usually when i he would be giving me a ride or something and she was in the vehicle.She was also on a sports team at our school but so was her brother. I want to tell her that I like her but I don't know how to talk to her brother about it. The last two boy friends were jerks and it didn't end well had her in tears. Any ideas on how to tell her and also how to talk to her brother.