Sibling Rivalry is episode 22 of family guy season 04. It was originally aired on March 26, 2006.
After a pregnancy scare, Peter reluctantly agrees to get a vasectomy. But after the operation, Peter’s sex drive becomes nonexistent. When Lois packs on some extra pounds, Peter’s interest is renewed and things heat back up in the bedroom until her unhealthy eating habits get the best of her. Meanwhile, Stewie’s half-brother and archrival Bertram confronts him and they declare an all-out war for control over the playground.