She didn't like her eldest daughter's boyfriend from the moment she met him. "I knew there was something wrong but I couldn't put my finger on it," says Catherine Kane, a west Belfast mother-of-three.

But she was determined not to interfere. When Nadine became engaged to Robert Mitchell on her 18th birthday, her mother bit her lip and joined in the celebrations. She even went shopping with her for a wedding dress.

But Catherine had other worries. Her two younger daughters were acting strangely. Lisa, 11, had grown increasingly distant, while the personality of nine-year-old Jennifer had changed completely.

"From being an outgoing child, Jennifer became very clingy. She cried every time I left her. But she had cerebral palsy and I blamed her behaviour on the illness," Catherine says.

On 2 August 2002, she came home from work to find Jennifer hysterical: "She was in her bedroom, banging her body off the radiator. She wouldn't stop crying. 'What's wrong, pet?' I asked her. 'When I go Robert's and Nadine's, Robert does things to me,' she said. I looked at her wee face and suddenly it all clicked."

Catherine ran to Lisa's bedroom, asking if Robert had 'hurt' her too. As Lisa started crying, the whole sordid truth came out. Robert Mitchell had been abusing his girlfriend's younger sisters for two years. He was convicted in 2004: Judge Tom Burgess sentenced Mitchell (20) to four years in a young offenders' centre.

Since the trial, the Kanes have tried to rebuild their lives. Then last week, their efforts were shattered. They were in Belfast High Court negotiating a settlement with the Compensation Agency.

"We were sitting in the court's big hallway when our barrister came over," Catherine says. "He pointed to Jennifer and said, 'They're offering you £25,000'. Then, he pointed to Lisa and said, 'They're giving you nothing.'

"Lisa was distraught. She left in a flood of tears. She feels completely devalued. Our barrister did his best but the Compensation Agency were so heartless. I don't know how they could offer one sister a payment and give the other sister sitting beside her nothing."

The Kanes refused the offer but worse was too follow.

"We've just received letters saying Lisa's still getting nothing and Jennifer's now being offered £10,000 – so because we refused their original deal they've cut her compensation by £15,000."

Lisa, who is now 18, says: "The Compensation Agency have treated me like I don't exist as a human being. We're not wealthy; I'd hoped to use my compensation to go to university to study to be a social worker."

Mitchell was Nadine Kane's first boyfriend. She was training to be a nurse. The couple looked after Lisa and Jennifer regularly. "They girls would go over to their flat or Robert and Nadine would babysit here if my husband and I went out for the night," Catherine recalls.

The girls later told their mother Robert abused them anytime he was alone with them: "If Nadine went to the shop, or for a shower, even when she was in the kitchen cooking, he'd be at them. He said that if they told anybody, he'd beat Nadine and that we wouldn't believe them anyway."

The abuse started when Jennifer was only seven. "He'd be tickling or wrestling me and then he'd touch me where he shouldn't," she says. "He'd pretend it was a game. He'd make me wear Nadine's underwear."

Once, he asked her to come into the shower with him and put out her hands while he masturbated into them. She didn't even know what he was doing. He made both children watch porn on his computer.

"It was disgusting; baby sex and something called 'seven-year-old pigtail sex'," Catherine says.

"The night the girls told me, I rang the police immediately. I was very calm. The next day it hit me. I cried for three weeks." Minutes after the girls disclosed the abuse, Catherine's husband rang Nadine.

"He said, 'Get out of that flat now, Robert's been abusing your sisters.' Nadine was shouting, 'No, no, he couldn't have!' But she left immediately. Robert tried to delete the images from his computer but the police retrieved them."

The girls gave their court evidence via video-link. "Jennifer sat there, nine years of age in pigtails, being cross-examined for 90 minutes," Catherine says.

"The pressure got to Lisa more. When they told her that Robert could see her, she shut down completely.

"She was sitting in a big chair in front of a camera. The chair seemed to swallow her. She didn't tell everything. She talked about the pornography but not the physical abuse. I think that's why they're refusing her compensation.

"I can't understand it. Both girls have been for psychologist assessment. The Compensation Agency has their medical and school reports. They must know what they've been through."

"I've found it hard to trust men," says Jennifer who is now 16. "I couldn't even be alone with a male teacher. In school, when we got sex education, I couldn't listen. I had to leave the class. I'm too sensitive. If I have a row with anybody, I'm in tears."

Even being in the courthouse last week unnerved her. She took an overdose of co- codamol days later.

"I have flashbacks and nightmares," Lisa says. "Sometimes, I think I see Robert on the street."

She still can't talk directly about the abuse and refers to what happened her in the third person.

Catherine recalls: "I caught her drinking when she was 15 and gave off to her. 'You've no idea what he did to me!' she yelled."

It wasn't just the abused daughters who suffered. "Nadine was suicidal for years," Catherine says. "She felt guilty for bringing Robert into her sisters' lives. She'd lock herself in her flat with the blinds down. I'd be petrified she was lying dead inside."

The Compensation Agency refused to comment on the Kanes' case except to say "each case is reviewed on its merits". The Kanes are preparing for a lengthy legal battle. "I had five miscarriages before I had a successful pregnancy," says Catherine. "I had IVF to have my girls. I fought to bring them into the world, and I'll keep fighting for them until the last breath in my body."

Victims' names have been changed to protect their identity