John Harbaugh, left, and Jim Harbaugh became the first siblings to face each other as coaches in a major sports match-up on Super Bowl Sunday in 2013. Older brother John Harbaugh's Baltimore Ravens won over the San Francisco 49ers.
Famous sibling rivalries – Probably the biggest (fictional) brother vs. brother rivalry took place on the popular 1980s TV show "Dallas." Bad boy J.R. Ewing -- played by actor Larry Hagman, right -- squared off against his good-guy brother Bobby, played by Patrick Duffy.
Famous sibling rivalries – The good-vs.-evil brotherly battle between Marvel comics characters Thor (Chris Hemsworth), left, and Loki (Tom Hiddleston) was the core of "The Avengers."
Famous sibling rivalries – Noel and Liam Gallagher's tumultuous relationship still triggers tabloid headlines today, years after Noel -- the calmer half of the brotherly partnership -- quit the British band Oasis.
Famous sibling rivalries – Esther "Eppie" Lederer, left, doled out advice under the name Ann Landers, while her twin sister, Pauline Phillips, authored the "Dear Abby" advice column under the pseudonym Abigail Van Buren.
Famous sibling rivalries – Biblical brothers Cain and Abel were the first -- and one of the worst -- examples of sibling rivalries. Cain's murder of his brother is depicted in this 17th-century painting by Bartolomeo Manfredi.
Famous sibling rivalries – Sibling rivarly was a running theme in the Smothers Brothers comedy routine in the 1960s. Tommy, right, often told Dick, "Mom always liked you best."
Famous sibling rivalries – Serena Williams, right, and Venus Williams have dominated women's tennis since they first went professional in the late 1990s. The sisters are close and often pair up during doubles competitions. But they've competed against each other in the finals of eight Grand Slam singles tournaments, including Wimbledon.