Mary Cheney, the younger daughter of former Vice President Dick Cheney who married her long-time partner Heather Poe last year, chastised her older sister, a Republican who is running for the Senate, for her opposition to same-sex marriage in a posting on Facebook.
Liz Cheney, who is seeking a Senate seat in Wyoming, said on Friday that she was "not pro-gay marriage."
"For the record, I love my sister, but she is dead wrong on the issue of marriage," Mary Cheney wrote on Facebook, according to the New York Times.
"Freedom means freedom for everyone," she wrote, repeating a phrase once used by her father when he was asked about gay marriage. "That means all families - regardless of how they look or how they are made — all families are entitled to the same rights, privileges and protections as every other."
Both Mary Cheney and Liz Cheney could not be reached on Sunday for comment.
Liz Cheney on Friday clarified her position on same-sex marriage in response to what she said was a "dishonest push poll" in which callers were asked whether they were aware that she supports abortion and aggressively promotes gay marriage.
"I am strongly pro-life and I am not pro-gay marriage," Liz Cheney said in her statement on Friday. "I believe the issue of marriage must be decided by the states, and by the people in the states, not by judges and not even by legislators, but by the people themselves."
Mary Cheney responded by saying that the issue was "not something to be decided by a show of hands."
Dick Cheney, who served as vice president under President George W. Bush, has long indicated he was supportive of gay marriage, saying that "people should be free to enter into any kind of relationship they want to enter into." But he has said states should regulate the matter, not the federal government.
Same-sex marriage is a divisive issue for Republicans. An ABC News/Washington Post poll in the spring found that a slim majority of Republican or Republican-leaning independents under age 50 supported gay marriage. In July, the Republican National Committee's chairman, Reince Priebus, reiterated the party's stand that marriage was between one man and one woman.