The sister of critically ill Sara Payne said last night her sibling was "a fighter, not a quitter".

Mrs Payne, the mother of murdered schoolgirl Sarah, was in a "stable" condition after battling complications from previous brain surgery, the hospital treating her said.

Her family has maintained a bedside vigil for Mrs Payne at St George's Hospital in Tooting, south London, since her collapse before Christmas.

It was reported she had endured two life-saving brain operations in one 36-hour period this week.

Mrs Payne's younger sister, Fiona Crook, 36, told the News of the World: "She's always been so strong. She's never given up. In my heart of hearts, I know she'll get through this.

"Sometimes you think 'how much more can one person take?' This woman has been through so much. She doesn't deserve this. I feel she'll come out of this. But, I have to be honest and say I'm not sure, and I'm prepared for the worst."

The 40-year-old was known to be responding well to treatment after being taken to hospital.

A family source has said she responded well to treatment and her condition has not worsened since she was admitted.

Friend Michele Elliott, founder and director of campaign group Kidscape, previously said: "Sara is the most wonderful and indomitable person and if anyone can get through this, it's her.

"Everyone needs to pray and send their good thoughts at this difficult time. I have been in tears."

Relatives and friends, including fellow campaigner Shy Keenan, have been at her bedside.

Mrs Payne, from Surrey, had a life-saving operation to cure a ruptured aneurysm last year.

Since the death of her daughter Sarah at the hands of paedophile Roy Whiting in 2000, Mrs Payne has become a prominent campaigner for victims' rights.

She launched a high-profile campaign for "Sarah's Law" after her daughter's murder, giving parents the right to know if paedophiles live near them.

A limited form of the law, based on America's Megan's Law, was announced in February last year.

Mrs Payne was given an MBE in the 2008 New Year Honours List for her tireless work to keep children safe from paedophiles.

Her daughters Charlotte, 15, and Ellie, five, are being looked after by relatives over Christmas. Her sons, Lee and Luke, are 22 and 21.

Her eight-year-old daughter disappeared while out playing in July 2000. Sixteen days later, Mrs Payne was told Sarah's body had been found in a Sussex field, around 15 miles from the cornfield near her grandparents' home where the little girl had been playing.

Whiting had previously served a jail sentence for abducting and sexually assaulting another eight-year-old girl.