When did Michael find out he had an half brother?

Michael found out May 12, 1991 that he had a half-brother named David Burchell. He was working on Noises Off and renting a house in Beverly Hills at the time. A reporter from the Daily Mirror called him about the brother he never knew existed. In his interview with Parkinson he talks about this:

Michael Caine explains: "[My mother] had an illegitimate child before she married my father. What happened was a newspaper was doing an article on mental health establishments in England.
"Unbeknown to me, my half brother [who was an inmate there] had a girlfriend who was a bit brighter than him - you couldn't understand what he said but you could understand what she said."
"The newspaper man got together a group of the more lucid, brighter ones - amongst which was my brother's girlfriend.
"Unbeknown to me, my mother had visited [my brother] every Monday - with the exception of the war - for 50 years…"
"My mother was dead by the time I found this out, she was gone for two or three years.
"I asked the matron at the hospital, 'how did this all keep quiet?' The matron said 'your mother used to bring a Bible and every new nurse had to swear on the Bible that this [boy] was not [your] brother.'
"She'd given him a picture of me from [the film] Zulu so he knew who I was when [he saw me] on the television. [My brother's] girlfriend told the reporter that he was [my] brother."
"I wanted to go and see him immediately. I went to see him. Do you know, the odd thing was that the name of the asylum was Cane Hill, on the other side of Streatham, but he'd been moved to another place by the time I found him."
"He died about 18 months after I got to know him. I went and saw him and had long conversations with him through the nurse because I couldn't understand what he was saying.
"My mother gave him to the Salvation Army to look after him... There was this incredible story."
Parkinson then asks Caine if his mother had told his dad? Caine replies: "Oh no, he'’d have killed her [laughs]!"
"I felt absolute amazement, how she'd fooled us all for years, every Monday. She used to come to the country every Sunday and my driver would drive her home the next day in the Rolls Royce.
"One day the driver said, 'she always gets out at the bus stop on Streatham High Road' and the penny never dropped, but she was catching the bus to the asylum round the corner.
"She'd buy and pick up chocolates, candy, cake and ice cream but when I'd go and see her, there was never anything in the fridge and I used to think, 'where's she putting where's she putting all this stuff? She was giving it to him. It's amazing."