what is it that you're looking for? i mean are you willing to go in a relationship with her or just want to boink her. cause if its just a boink then get the fuck out... if its more i suggest you talk to the sister but not straight up. just chill with your friend as usual and ask her to invite her sister. Then after a couple of times doing that just talk to your friend about her sister and shit.

That will give you time to get to know the sister and see if you could go in a relationship with her before you risk your friendship for someone whos not orth it.