Sister Act: Dick Cheney's daughters argue publicly over gay marriage The civil war in the family of former vice president Dick Cheney between his two daughters, quietly building over the summer, has escalated into a very public dispute. It's becoming one of the highest-profile, and deeply personal, political debates so far over same-sex marriage in the US. 47-year old Liz Cheney is at odds with her younger openly gay sister, Mary (44). Liz says she "disagrees" with her sister's right to marry. In an interview on "Fox News Sunday," Liz Cheney argued against the discrimination of people "because of their sexual orientation," but then portrayed herself as a staunch social conservative: "I do believe in the traditional definition of marriage." The background: Liz Cheney has entered a tough primary fight to unseat long-time senator and fellow Republican Mark Enzi in her adopted home state of Wyoming next year and needs to double down on her conservative credentials. Her interview on Fox got the attention of Mary Cheney and her wife. Both women decided to not hold their tongues on the disagreement and fired back in an extraordinary way - via social media. 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. To have her now say she doesn't support our right to marry is offensive to say the least," Mary Cheney's wife, Heather Poe, wrote on Facebook. Mary Cheney then shared Poe's remarks on her own Facebook page. Liz - this isn't just an issue on which we disagree - you're just wrong - and on the wrong side of history. Mary Cheney has publicly rebuked her sister's position on gay marriage in the past. "For the record, I love my sister, but she is dead wrong on the issue of marriage," she wrote on Facebook in September. In any other family, this would be an awkward, yet private debate, but not in the family of Dick Cheney, for a long time one of the Republican Party's stalwarts of no-nonsense conservatism. Yet, given the sexual orientation of his daughter Mary, Cheney endorsed same-sex marriage in 2009, alienating himself from parts of the right wing of his party. Cheney's obvious change in position created a pattern for other Republicans whenever they were confronted with a gay family member. In March, conservative Ohio Senator Rob Portman announced that he would support the legalization of same-sex marriage two years after he learned that his son was gay. And just a few days ago, Portman was one of the few Republican senators to vote in favor of legislation that makes discrimination of gays and lesbians at their workplace illegal. Political analysts believe that the Republicans will need to come to terms with the reality of modern life in the United States, if they want to be considered a serious political force in the future. The rejection of same-sex marriage is considered by an increasing number of Americans as a position of the past.