what would you do if u have fallen in love with your gf/bf's sister/brother?

First, I would have to break up with my current boyfriend. If I fell in love with ANYONE else, that is the first order of business. I could not move on to another relationship before ending the one I was already in. Obviously, if I love another person, my current relationship is not satisfying me or fulfilling. Probably for either of us. It would be the ultimate in selfishness to do anything less. If it was his brother, I would probably have some reason to feel that way such as from mutual flirting or deep conversations. Then I would gauge whether or not the brother felt the same way and pursue this the same way I would pursue any other relationship. I'd have to find out if he was interested in pursuing a relationship with his brother's ex-girlfriend. If we did mutually decide to pursue a relationship I would want to keep it secret for awhile before going public with it to his brother or anyone else. Then I would try to break it to the brother gently. I don't know what else you could do in this situation. Otherwise it might get pretty "Maury Povich or Jerry Springer-ish" if you know what I mean.