Friend!

you r siter has fallen out with her best friend and u fancy that friend would u ask her out knowing that thereis something special there?

Depends on your relationships with the girl and your sister. I met my boyfriend through my brother; they were friends, he asked me out. It has worked out exceedingly well in our case, but you don't want to ruin your sister's friendship.

Talk to your sister and see what she thinks... if she wants you to stay away from her friend I think you should respect that.