On December 19 1998, my sister Surjit didn't arrive home as planned from a last-minute holiday to India. At the time, her marriage had acrimoniously collapsed and divorce lawyers had been instructed. Yet strangely, amid that atmosphere, she had gone on holiday with her mother-in-law, Bachan Athwal.

I phoned Surjit's estranged husband, Sukhdave, who didn't seem concerned. "She's probably run off with a boyfriend. She's prone to running off with men - she'll be back." I called airlines and the Foreign Office without any luck. I was in an abyss.

It seemed strange that Surjit had left her two children behind and I couldn't help fearing she had been the victim of an honour killing. But I held on to the hope she'd just gone off for a few days alone. I knew Surjit's life had been difficult with her husband and his mother-in-law but I still couldn't grasp the idea that they could be murderers.

Eventually, I went to Surjit's house in Hayes and talked to her husband, who kept changing his story. I became suspicious and went to the police but they turned me away; they said they didn't deal with disappearances abroad. Frustrated, I approached my local councillor. Within days, he had organised a meeting with the Special Crime Squad at Scotland Yard and they began formal investigations.

Months passed and we didn't hear anything. Then, one Saturday, a detective called to tell me that my sister was dead. The police had received an anonymous letter saying Surjit had been strangled. But, just weeks later, the police - having spoken to Surjit's husband - told us they were not convinced she was indeed dead.

The case was going nowhere. Then, a few months after that, my father received two letters and two calls from UK phone boxes. An anonymous voice described how Surjit had been killed by two men, whom the voice named, and had been thrown into the River Ravi in Punjab. The killing, the voice said, had been orchestrated by Surjit's husband and mother-in-law.

Until that moment, I had always visualised Surjit walking through the door and the whole family being happy again. But now, all I could think about was getting justice for her. I never had a spare moment to rest or mourn. I was always writing letters and putting pressure on the lacklustre police.

But the police case stopped and started, witnesses wouldn't go on the record and by 2003 I was emotionally, physically and mentally exhausted. I began to question how much I belonged to Britain. I saw, for example, the government's immediate and public action over Lucy Blackman's disappearance in Japan and I was bitter. Why didn't that happen for Surjit? I felt I must stop and leave it to God.

The turning point came in September 2004 when Newsnight made a 10-minute film about Surjit. They talked to the main suspect and to the British High Commissioner in India who said the case had been left to drift. I felt vindicated. But the real luck came when one of the witnesses saw the film and finally agreed to go on the record. The witness had been at a secret meeting in which Surjit's murder was planned.

In November 2005, police arrested Bachan and Sukhdave. They found Surjit's passport and diary in their attic, as well as forged documents signing over Surjit's house to her mother-in-law. They also found forged Metropolitan Police letterheads that had been used to disrupt the investigation.

The pair were charged with murder and, after a 13-week trial this year, were both found guilty and jailed for life. The two men who carried out the murder in India, however, are still free. Honour killing is now taken more seriously in Britain than it was nine years ago and Surjit's case has been instrumental in bringing about that change. It's little comfort, but I think that at least my sister's death was not completely in vain.