first off talk to your friend and make sure your ok with it
also ask them if she is already seeing someone
if shes single just try and talk to her. get to know her better. compliment her
develop a friendship with her and then go from there

alright then make a move
try and ask her out

Go in for the kill, but warn your friend that you're gonna do that first.

It's respectful to give him some warning that you plan on scoring with his sister. If he gets really uneasy with the idea, do reconsider.