This one is for dudes with younger sisters?

I can fully understand your dilemma and appreciate your concerns for your brother. The biggest problem with dating someone who is an extremely good friend of your brother's is that if you try for a relationship with this other person and it doesn't go well it could easily impact the association between the friend and your brother. Your actions could cost your brother his very good friend. If you should decide to go forward with this idea I would suggest it be only after you have really talked over the possible implications with your brother. You don't want to be the one who ruins a friendship your brother has had for over fifteen years. Go slow.