Siblings who live together should have the same rights as gay couples under new laws, some Conservatives have said.
The move is seen as a tactic to try and trip up the legislation
They want brothers and sisters living together for at least 12 years to
have the same rights being proposed under the Civil Partnership Bill.
Lord Tebbit and MPs Gerald Howarth and Edward Leigh said the Bill should be
fairer to "ordinary families".
The Bill, before MPs and Lords on Tuesday, gives same-sex partners similar rights to married couples.
At the moment gay people may have no claim on the property they share if their partner dies and they are not registered as a legal owner.
The Bill does not extend to heterosexual unmarried couples.
CIVIL PARTNERSHIP RIGHTS
Social security and pension benefits
Possible parental responsibility for partner's children
Full recognition for life assurance
Responsibility to provide reasonable maintenance for partners and children
Same tax treatment as married couples, including exemptions from inheritance tax on homes
Visiting rights in hospitals
Mr Leigh has tabled an amendment to extend the Bill to brothers and sisters who live together. The move is likely to be seen as a wrecking amendment.
A full-page advert backing the move and commissioned by The Christian Institute will appear in The Times newspaper on Tuesday.
MPs approved the Bill at its second reading by 426 votes to 49 before it went to the committee stage for consideration.
The plans, announced in last year's Queen's Speech, follow a long campaign for equality for same sex partners.