Time, they say, is a great healer. But how you come to terms with the murder of your adored elder sister in a faraway country in Africa, torn asunder by civil war, by vicious and faceless rebels with whom you share absolutely nothing in common, is far from straightforward.

Do you fester in hatred and seek revenge? Do you wallow in pity? Do you forgive? Do you blank it out? Or do you seek answers and justice? For much of the period I have known him, Richard Wilson, a 30-year-old Londoner, has had to cope with precisely these issues. Today, he tells me he thinks he has found most of the answers after more than four years of painful investigation. But no justice.

He is still grieving and he has written a book about the tragedy: not comfortable reading. But then there is nothing comfortable about the way his sister Charlotte Wilson died. Three days after Christmas 2000, she was killed in Burundi when the bus she was travelling on was ambushed by Hutu rebels, crashed and overturned in a ditch. She was 27.

Charlotte was teaching at a school in neighbouring Rwanda and had recently fallen in love with a 6ft 6in-tall Burundian teacher called Richard Ndereyimana, a former monk. Just a few days earlier the couple had got engaged. They were on their way to celebrate the New Year in Burundi with his parents.

The rebels robbed the passengers of the Titanic Express and ordered them to lie on the ground. Then they opened fire. Her fiancé and 19 other passengers, including children, were killed. Charlotte died with seven bullet holes in her back.

She was blissfully happy, according to her last letters home. “I’m definitely following my star, even if it leads me to places that aren’t part of the conventional road to happiness,” she wrote to her mother.

She was happy, but she was also much affected by the recent genocide in Rwanda, which still cast a shadow over the land. One of the beliefs she held was that it was wrong to target people because of a label. In the 1994 genocide in Rwanda, hundreds of thousands of Tutsis had been slaughtered by Hutus. The Hutus had massacred the Tutsis not as individuals but because they bore a Tutsi label.

Tragically, as her brother has found in his investigations, his sister, too, was very likely seen as a label, not as an individual, by her killers. The rebels labelled her as a white person and to them that was a crime because, as one of them who shot her said in justification before he pulled the trigger on his AK-47: “It is you white people selling the weapons in Africa. Now you are going to feel what it is like.”

“So she had to pay the price for the arms trade,” Wilson says. “The gun that killed Charlotte was probably supplied by a European dealer.”

No family has been more remote from the arms business than the Wilsons. Margot and Peter Wilson were schoolteachers living in Italy when Charlotte was born. Moving back to Britain they had two more children, Richard and Catherine, but Peter died of leukaemia when Charlotte was six. Widowed, Margot never remarried, but devoted herself to her children.

She instilled in them a deep sense of morality, decency and independent thought. Her idealism rubbed off on Charlotte and with such a background it was the most ordinary thing in the world for her — after taking a PhD in microbiology — to leave Britain in 1999 to do voluntary work in Rwanda, one of the most forsaken countries on earth, and put a promising scientific career in London on hold.

Wilson will always wonder why his sister was travelling on that dangerous road when everyone advised westerners against it. But her father’s death when she was six had hit Charlotte just at the age when she was old enough to feel the force of it but did not have the emotional resources to deal with the loss. Even in her twenties she spoke of her grief as a “black cloud” that was always there. It left a hole in her life that could never be filled. Wilson wonders, still, whether it was that “black cloud” that had obscured her judgment so disastrously over Burundi.

A few months before her murder he had visited her in Rwanda and worried that she was reckless about her safety. What worried him most was that she seemed so willing to put her own safety at risk to prove a point. “My sister could be bossy, irritating and stubborn,” he says. But she was also the “most generous, gregarious, good-hearted and energetic person I knew”.