"Sibling Rivalry" is the 22nd episode of the fourth season of the animated comedy series Family Guy. It originally aired on Fox in the United States on March 26, 2006. The episode follows Stewie as he battles with his half-brother, Bertram, who is born to two lesbians after Peter donates sperm. Meanwhile, Lois begins excessive eating after Peter undergoes a vasectomy and does not feel well enough to have sex.

The fourth season of Family Guy aired on Fox from May 1, 2005, to May 21, 2006, and consists of thirty episodes, making it the longest season to date. The first half of the season is included within the volume 3 DVD box set, which released on November 29, 2005, and the second half within the volume 4 DVD box set, which released on November 14, 2006. Volume 4 is split into seasons 4 and 5 in regions outside the United States, leading to confusion over season numbers between U.S., Australian, and UK consumers. The last three episodes of season 4 are the basis for the movie known as Stewie Griffin: The Untold Story, and are edited for content; Fox does not include these episodes in the official episode count.