Depends on the sister and the friendship.
Ive hooked up with a really good friend's sister at HIS wedding. Lmao. Attraction was there from start of the evening and she made an excuse of going to an "after party" when in fact she just came a few hotel rooms down the hall to mine. The friend never found out. He has a massive temper issue and he is considerably larger than me, so I'd be afraid he'd beat the crap outta me. LoL
Another good friend's sister was super hot in high school but out of my reach. However as she approached 30 now, the wall hit and everytime I go over to his parents house for a formal dinner or CHristmas party or when she's out with my friend and his wife, she always gives me admiring glances and tries to flirt.
Needless to say though, I wanted the 21 year old version of her, not the 30 year old version. And I think if my friend found out, it would compromise our relationship so I've stayed away.