Help.
I'm in love with my sisters best friend,we are totally compatible and my sis is OK with it,we talk for hours and have loads of fun but its never gone any further,what do I do now?

Ask if you want to be more then just friends. There is always a risk of revealing your feelings but it is the risk you should take no matter what. Everything is a learning life experience. Take the chanch it could lead to even better things. Remember though there is never any guarantees.

Assuming you are old enough to date ( and it does not sound like it, or you would know what to do) but you ask her out.