Supporters of gay marriage hope confirmation that Tony Abbott's sister is in a lesbian relationship may convince Federal MPs to back their campaign.
But there appears a hardening position within the Liberal Party that it cannot countenance any change, in part for fear of undermining its efforts to paint Prime Minister Julia Gillard as a liar on the carbon tax.
It comes after a story at the weekend on Mr Abbott's sister and her lesbian relationship.
Christine Forster, 47, is the youngest of Mr Abbott's three sisters.
Previously married and with four children, Ms Forster came out as a gay woman in 2008.
She met Virginia Edwards, who had sons at the same Catholic school in Sydney's northern suburbs as Ms Forster, and now lives with her.
Despite her high-profile brother, Ms Forster had sought to keep publicity about her relationship at a minimum.
But a photograph of her holding a sign saying "I Love Her!! Marry Me?" with Ms Edwards in Sydney last year has been posted online.
Mr Abbott said that he and his sister had had "discussions" rather than arguments about his view on gay marriage.
He also conceded that while he respected his sister's choices he could not in "every sense understand it".
"She's a very level-headed, down-to-earth, pretty tough-minded individual," he was quoted as saying.
"She's more politically centrist than I am but she's no bleeding heart lefty either."
Australian Marriage Equality spokesman Alex Greenwich said the story of Ms Forster helped explain to opponents of gay marriage the pain their stance caused.
"Advocacy from loved ones is by far the most effective way of changing hearts and minds," he said.
"Ms Forster proves the point that this reform is about the importance of family, love, and equality, not politics."
A Senate inquiry into a private members' Bill to allow gay marriage has been inundated with public submissions. It has received so many that it will not be able to publish all of them online, while it has a series of public hearings to conduct around the country.
Mr Abbott has hailed his sister's bravery for coming out but says it will not shift his view.
"We've had a lot of interesting discussions and we'll keep those discussions going, but fundamentally I want to be a politician that keeps my commitments," Mr Abbott said.
"We went into the election with a position and that's the position we will keep."
Shadow attorney-general George Brandis said the Opposition shadow cabinet had decided in 2010 not to support a conscience vote on gay marriage. In an echo of the Opposition's attacks on Ms Gillard, he said yesterday that it was important "that promises be kept".http://au.news.yahoo.com/thewest/a/-/national/13372623/abbott-firm-on-lesbian-sister/
