Well, I can tell you the experience from the first hand - one of my female colleagues used to date a guy, whom she had a baby with later. However, she eventually found out she was attracted to his brother, so she broke up with this guy and married his brother, whom she has a baby with too. The guys have stopped meeting since then, the animosity between them is apparent and even understandable. Who would like to be cuckolded by his own brother in the extent where babies and marriage are involved?
Somebody should say it is all the same genetic line, so it really doesn't matter which one of the siblings is getting the particular desired pussy because the same genes will be passed on.
I don't agree however. I see an atomic family and close relatives as the unit who facilitates the survival of the individual through cooperation, but the reproduction of the individual is always the most selfish and personal act above all the aspects of the cooperating and familiar (genetic) propinquity. As such, the animosity and rivality between siblings is a necessary output (interpret it as an affection and cooperation among the players of the particular footbal team to get higher chances win. Even withi such settings, players always tend to exhibit their individual qualities and be the best, regardless the rest of the team).