I have a growing crush on a close friend's sister.
Why do you feel you have a crush?
A bit of an overactive libido.
If you have feelings for this person you might want to think about talking to them about it. Have you tried to talk to them and see what they feel about getting together with you so that you might be able to resolve some of that crush?
We have never talked about anything personal.
Well you don't have anything to lose if you tell them your true feelings. If it isn't meant to be it won't and you can move on.