The call came in the middle of rehearsals for a play David was directing in New York. Back in 1991, that was meant to be his first proper break. Now all he can remember about it is his aunt's English voice on the end of the line. "Something's wrong with your sister."

David cancelled the play and drove to the airport. He called his father in Vermont and told him the news. Hannah was in hospital. A psychiatric ward. And there was something else. "I dunno how to say this, Dad, but she's claiming that you, uh, sexually abused her ... Yes, it sounds insane, but what shall we do?"

"I don't know. You talk to her."

Then David called his mother in LA. She hadn't spoken to her daughter for 10 years, but she'd just heard the allegations. "And I believe her," she said, without hesitation. David felt himself turn cold. "There are things you don't know," she added. "I'll tell you in London."

It was the most upsetting sight David had ever seen. His sister was like a "boiling cauldron of hatred". She had been hallucinating for a week since returning from a trip to India and taking LSD, which had induced a prolonged psychosis. In between paranoid fits, Hannah would become momentarily lucid, and tell David what she'd begun to remember: her father violently raping her when she was a teenager, and when she was a baby.

David and Hannah were born twins, but grew up strikingly different. They made an odd pair: the troubled, uncontrollable sister and the logical, nerdy brother. David thinks that his own ordinariness must have been a reaction to the "almost cinematic, 1970s, New Age wackiness" of his parents' lifestyle. Their fads included Scientology, transcendental meditation, vegetarianism, Buddhism and Jungian psychoanalysis. When Hannah flew into one of her childhood rages, her parents would veer between alternative therapies and locking their daughter in the toilet to calm her down. After their divorce, Hannah disowned her mother, moved to England, and would speak only to her father.

Until she began to recover memories of him abusing her. In London, her mother briefly got to reclaim the role of saviour. She railed at the hospital psychiatrists, informed them her daughter was having a "spiritual emergency", took Hannah off her anti-psychotic medication and booked her in for sessions with a Jungian analyst. She told David suspicious things he hadn't heard before - about his father's depressions, how he once waved his penis in front of Hannah, how she had demanded a lock for her bedroom when she was 12.

As far as David was concerned, his mother was as unreliable a source of information as everyone else in his dysfunctional family. The mid-1990s were the high period of the "recovered memory syndrome" scandals in the US, when therapists would talk patients into retrieving fictional tales of childhood abuse. The accusation being fired against the recovered memory therapists was that of "iatrogenesis" - the moulding of a patient's trauma by an incompetent doctor's own ministrations.

Psychological research increasingly demonstrated that the Freudian concept of repression was a sham. It is extremely rare for a childhood memory to be buried and then retrieved years later in all its original clarity. Most people who have been abused over the age of five or six do not forget it. Those who, like Hannah, claimed to remember what happened to them as babies were proposing a neurological impossibility. It didn't help the credibility of her story that she also persisted in claiming she was having a telepathic relationship with a boy in India.

Perhaps, in Hannah's case, the LSD had precipitated its own iatrogenesis, leaving her unable to distinguish between hallucination and memory. David has no way of knowing. "My dad certainly had a lot of concealed aggression, negativity and darkness. When Hannah used to describe to me what she thinks happened, she sounded so distressed, I believed her. But my sister was a traumatised person. Her allegations of sexual abuse might be symbolic of her trauma, or abuse might have been the cause of it. There's no proof either way."

As a theatre director, David finds himself drawn to characters who are plagued by doubt. He sees dramatic conflict as a search for what psychoanalysts call the primal scene. But he utterly rejects Freud's seduction theory, which gave priority to subjective experience and broke down the distinction between real and imagined trauma. David has his own terror. Aged 40, he is still attracted to much younger women. Does that have something to do with his twin sister and his father? For David, the facts matter, even if they cannot be known. The question of whether or not something really happened makes all the difference in the world.

· Names and details have been changed