I see no problem with it so long as everyone acts mature about it. But there can be risks to it. Ya may want to ask yourself if your friendship would ever be affected by it should you break up with the other person. Be sure to treat your friends sibling with respect (of course, you should treat anyone you date with respect anyway), at least, and you should be OK.

 I think it's ok as long as your bro or sis doesn't have a crush on his or her friend. Make it feel as if you are not interfering.

 Why would that be wrong??

 It really all depends. If you and your bro/sis are tight like bffs then go for it.

 I've been stuck in this problem too but try having your big bro being the captain of the wrestling team as well as the friend. I have this huge crush on my bro's friend forever and he thinks I'm cute but is completely afraid of my bro. But when my bro's girlfriend (now my best friend) found out i liked him she told her mom who called him up and told him to stop being so stupid and then told my bro to tell his friend that its okay to date me.
So i think its alright as long as everyone is mature and there's no real drama.

 Go for it