I get stuck in the friend zone alot,  the one thing I have to say is sometimes we see what we want to see.  Im not saying she dosent like you but be prepared for if she dosent.  Play it cool and be careful because when you fall hard and fast it could real end up hurting.  As far as your sister you need to be honest because if your friend finds out from somone other than you or her friend things could get really ugly.  Honesty is always the best policy, ecspecialy when it comes to family.