When Darline Graham was appointed last month to fill the South Carolina Senate seat of her late brother, Lindsey Graham, she became the first sister to directly succeed a sibling in Congress.
Family dynasties are common in American politics. Elected offices all the way up to the presidency have been held by one relative after another. About 30 senators have served simultaneously with a sibling in Congress. And more than 40 women, including four in South Carolina, have directly succeeded their late husbands in Congress.
But sibling successions are rare.
Ms. Graham, a disability rights advocate, had a close relationship with her brother, who never married or had children. Mr. Graham was seeking a fifth term in the Senate at the time of his death in July at 71. On Tuesday, his sister is facing Representative Ralph Norman, a longtime conservative lawmaker, in a special Republican primary runoff for the nomination.
Ms. Graham has never held elected office, and questions about her political inexperience have grown louder since she acknowledged in a recent televised debate that she was “not that informed on national security.”
Still, her name recognition and family ties give her campaign a time-tested “leg up,” said Brendan Nyhan, a political scientist at Dartmouth College.
“It gives you a shot you might not otherwise get,” Mr. Nyhan said. “We often have these parasocial relationships with elected officials. We feel like we know them, and that may carry over to their relatives.”
In the race for Texas’ solidly red 22nd Congressional District, Trever Nehls is the Republican nominee to replace his retiring twin brother, Representative Troy Nehls. Both Ms. Graham and Mr. Nehls have the endorsement of President Trump.
Several congressional seats have been directly passed down from brother to brother.
Representative Brian Fitzpatrick, a Republican and former F.B.I. agent, moved to Pennsylvania from California and first won the House seat left open by his retiring older brother, Michael, in 2016.
Representative Mario Díaz-Balart, a Florida Republican, switched districts in 2010 and won in the more Republican-leaning district that his brother Lincoln Díaz-Balart had represented since 1993.
Asa Hutchinson, the former Republican governor of Arkansas, got his start in Congress in 1997 after his older brother, Tim, gave up his House seat for a successful Senate run.
Morris K. Udall won a special election in 1961 to fill the Arizona congressional seat vacated by his older brother Stewart, who was appointed as interior secretary by President John F. Kennedy. Mr. Udall, a prominent liberal voice in Congress and one-time Democratic presidential candidate, held the seat until his retirement in 1991.
And in 1946, Sam J. Ervin Jr., a Democrat, won a special election for the North Carolina congressional seat left open after his brother Joseph died by suicide. Mr. Ervin would later become a senator and lead the Senate’s Watergate Committee, whose inquiry led to the resignation of President Richard M. Nixon.
Siblings in Congress were common in the late 18th and early 19th centuries, but rarer around the Civil War and even more rare over the last hundred years, according to James Feigenbaum, an economic historian at Boston University who has studied family connections in Congress.
Senator Willard Saulsbury of Delaware sought re-election in 1871, but as concerns grew about his erratic behavior, he faced a challenge from not one but two of his brothers.
Eli Saulsbury, the winner, remained in office for the next 18 years.
