Mary Cheney, a daughter of former Vice President Dick Cheney, and her wife, Heather Poe, sharply criticized Sunday a comment by Liz Cheney, a candidate for the Senate in Wyoming, that she and her sister disagree on the issue of same-sex marriage.

“Liz — this isn’t just an issue on which we disagree, you’re just wrong — and on the wrong side of history,” Mary Cheney, who is gay, wrote on Facebook.

Poe’s comments were sharper and more personal.

“Liz has been a guest in our home, has spent time and shared holidays with our children, and when Mary and I got married in 2012 — she didn’t hesitate to tell us how happy she was for us,” Poe wrote on her own Facebook page. “To have her say she doesn’t support our right to marry is offensive to say the least.”

Appearing on “Fox News Sunday,” Liz Cheney, who is challenging Sen. Mike Enzi, said she and her sister parted ways on whether gays and lesbians should have the right to marry. “This is just an area where we disagree,” Liz said.

Her sister’s salvo on Facebook is the second time this fall the younger Cheney has criticized her sister’s position on same-sex marriage. The comment suggests that both Mary Cheney and Poe are going to continue speaking out on the issue during the primary.

Such comments could present political difficulties to Liz Cheney, who is attempting to unseat Enzi by running to his right. Dick Cheney has already said that he supports same-sex marriage, but the matter is now plainly causing family friction. The back-and-forth comes as Liz Cheney ramps up her campaign and Dick Cheney is back in the news promoting a book he wrote with his elder daughter about his heart transplant. The former vice president is also playing a role in Liz Cheney’s campaign — he is joining her this Wednesday in Denver for a fundraiser.

In her post, Poe argued that the patchwork of same-sex marriage laws is unfair to gay and lesbian couples. And she raised the issue of Liz Cheney’s move last year from Washington to Wyoming, a sensitive issue in the state and one Liz Cheney sought to address by touting her family’s state roots in her first television ad last week.

“I can’t help but wonder how Liz would feel if, as she moved from state to state, she discovered that her family was protected in one but not the other,” wrote Poe, who called Liz Cheney “my sister-in-law,” adding: “Yes, Liz, in 15 states and the District of Columbia you are my sister-in-law.”

Mary Cheney praised her wife’s statement — “couldn’t have said it better myself.”