Liking My Friends Sister...?

Hi, I sorta have a problem. My best friend who is the bassist of our band, I'm lead guitarist, well his younger sister is very beautiful to me, when we're talking i cant help but stare into her eyes. I am a little bigger than most guys (in weight). im age 15, 220 pounds, im 6 feet tall and she is age 14, 95 pounds at the most, maybe around 5 foot 6 inches. Im a sophomore in high school and she just graduated 8th grade and became a freshman. I really like her, i always get off track live when im playing especially when i look into the crowd of like 500 and see her smiling at the band, yes i only notice her. I always get a funny feeling when i see her. Well, she is my best friends sister and it seems kinda awkward to ask her out, i dont even know if she likes me but its a chance im willing to take. How should i approach my friend? I really like her, a lot, is it love? I have had crushes on girls but with her, everyday i think "hmm, i wonder what shes doing, i will txt her" and every morning i txt her and say goodmorning, how you feeling?. Help please