The government has staved off a confrontation in the House of Lords on plans to give gay couples many of the same rights as married people.
Germany introduced similar equality legislation in 2001
A Conservative peer proposed offering family members living together some of the rights offered gay couples under new civil partnerships.
The proposal would have set up a battle of wills with MPs but peers voted down the idea by 251 votes to 136.
The Civil Partnership Bill is now set to become law this week.
Peers agreed other amendments proposed by MPs without any votes.
The controversial amendment by Tory Baroness O'Cathain called for close family members living together for at least 12 years to be able to get the same rights for capital gains and inheritance tax, fatal accident claims, and housing tenancies.
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
Visiting rights in hospitals
Some MPs last week accused those behind a similar move in the House of Commons of trying to wreck the Civil Partnerships Bill by making a point about gay marriage.
If peers take the opposite view, it would open a battle of wills in the final day of this session of Parliament.
Lady Scotland said she had sympathy with the aim of the amendment but urged peers to reject the idea to prevent the Bill being "held to ransom".
Former Labour Health Minister Lord Hunt said the change would make the legislation unworkable.
But Lady O'Cathain said excluding siblings from the Civil Partnerships Bill risked creating a new injustice.
If gay couples were to get new rights because they could not marry, siblings who could not marry should also get new rights, she argued.
She spoke of "special rights given to some but not to others who are in equal, if not greater, hardship".
Lady O'Cathain said the parallel scheme for family members would be "very much limited".
She acknowledged ministers' arguments that the bill was not the right legislation for the change.
But she declared: "We may never get the right bill."
Former Conservative Cabinet minister Lord Tebbit said the proposal was not a wrecking measure.
And he said it would not be needed if ministers would promise not to give a tax advantage in any future Budget to civil partners unless it was shared by family members who lived together.
The bill would create a "new inequality" between large numbers of co-habiting parents, children and siblings and the small number of civil partners, he said.