My cousin died, aged 61, within four minutes of her life support machine being switched off. Inquirers were told that she died of “natural causes”. No one asked more. After all, she was mentally handicapped and they always die early, don't they? In fact, she died a degrading, unnecessary and slow death, whose cause is known only to her siblings - and me. Maria died of untreated syphilis.

I knew she had it, but I ignored my conscience and did nothing. This will be a guilty stain on my heart all my life. I should have shouted it loud and clear, years ago, when her siblings retreated, baffled by the system. I have a skill with officials and often help friends. But faced with Maria, well, I wimped out.

The truth is, I couldn't face the stomach-turning horror of Maria's personal appearance and aggression - or the smell - and I kept my head down until she died.

Maria was born “backward”, as they called it in the 1940s, after her mother, my Aunt Sylvia, caught rubella during her pregnancy. She was damaged in the womb, but not hopelessly so. A sturdy, cheerful girl, she could read and write when she left school, and looked forward to life with zest. But the menial office job she landed was soon abandoned, because her father Jack, a bank officer, felt it was beneath the family.

So a wall of thorns grew up around Maria. She lived at home with her mother, who was too proud and ashamed to seek help. While she never developed men- tally beyond the age of 8, her sexual development was normal. I recall her coming out of the bathroom, her skirt tucked into her knickers and a blood-stained sanitary towel visible.

“I'm not married yet,” she would say, smiling optimistically. I hoped that she would meet someone who wanted a good-natured, loving, rather childlike wife. As time went on, and her father died, I thought: never mind, Maria is living with her protective, devoted mother and sending me postcards from their holidays in Spain. But years later, after Sylvia died, a different picture emerged. Maria did not appear at Sylvia's funeral. She was wandering around the port town where they lived, telling anyone who would listen, “I'm free at last”.

Not that certain men needed telling. As old age encroached, it seems that Sylvia gave up trying to control her feisty daughter. Maria was allowed to wander the streets and - from clues she dropped about “men in cafés” who were her “special friends” - to find her way to the docks. A good-natured innocent with a woman's body. What a gift for any Tom, Dick and Harry looking for casual sex before sailing away.

Sylvia must have known that something sexual was going on. Maria told one of her sisters that Mum took her to have an operation “down below” years earlier. We all guessed that she had been sterilised.

Maria now lived alone. One of her brothers paid her bills but few people actually visited her. Family “How do we solve the problem of Maria?” meetings became more urgent when one sister reported that Maria developed weeping sores on her body.

Both sisters took it in turns to take her to the GP. But because Maria was responsible for herself, they found themselves barred from the consultations. The doctors insisted on seeing her alone, and goodness knows what Maria told them. She simply reappeared and her sores remained undressed. Anyone's requests for information were greeted with official vagueness.

I should have gone up there and hammered on the GP's door. Maria, meanwhile, went from chubby to stick-thin. She lost her hair, developed a rash over her head - and her phone calls stopped when she went deaf. Her sight was fading, too.

Once, one of her sisters made a 120-mile journey from home to take Maria on a “showdown” visit to the GP. She emerged, feeling triumphant, with a chit for Maria to have a “brain X-ray” at a local hospital. Seven hospitals later, she was told that this process was now obsolete. So Maria went home and her sister drove home, exhausted and baffled.

Her other sister took over. She Googled her symptoms and only one diagnosis came back that covered everything: syphilis, in the final stages. It made perfect sense - the sores, the rash, the hair loss, the deafness and blindness, the angry episodes. So why didn't the doctors do more? We'll never know.

Maybe they could not face dealing with Maria either. Because now, my once-smiling cousin shouted and lashed out all the time. Maria was barred from the post office and, heartbreakingly, from the little local bus to the shops that she loved.

Finally, her brother found her in a coma in bed after a Christmas she had spent alone, not eating. The last mystery is why the hospital struggled to keep her alive on a machine for five days - until her sisters begged them to turn it off.

At the funeral, Maria's brother recited Larkin's poem: “They f*** you up, your mum and dad.” In Maria's case, they're not the only ones.

Do you live with a family secret?

How has it affected your life? Do you still struggle with it? E-mail us at familysecrets@thetimes.co.uk

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