“For the record, I love my sister, but she is dead wrong on the issue of marriage," she wrote, according to the New York Times.
Mary Cheney's post came in response to Liz Cheney's reaffirmation of her opposition to gay marriage and abortion in a statement from her campaign issued on Friday.
Liz Cheney is challenging incumbent Sen. Mike Enzi (R-Wyo.) for the Republican nomination in Wyoming's Senate race.
The campaign said Cheney was prompted to clarify her position on the two issues after she became aware of a "push poll" currently being conducted in the state that allegedly suggests Cheney "supports abortion and aggressively promotes gay marriage."
Liz Cheney's position puts her at odds with both her sister, Mary, who is openly gay and last year married her longtime partner, and her father, former Vice President Dick Cheney, who came out in favor of gay marriage in 2009.
Mary wrote in the Facebook post responding to her sister that “Freedom means freedom for everyone.”
“That means that 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,” she said, and called for her Facebook friends to "like and share if you agree."