Maria: Sonia is one of the great blessings of my life. As a child, I just adored her, and I still do. She was born when my parents were in the process of separating — a tiny, beautiful thing needing love and care, who became a great distraction and saved us from the terrible sadness of that separation.

My parents were academic and huge achievers, which made me feel like a failure. Mum had to work after Dad left. She tried to get au pairs, but they'd arrive  and we children would stand glaring at them, saying: "We don't want you."

We were a self-contained unit. We didn't have a television and we were left to our own devices. We'd all get into one bed and Richard would make up stories, and we spent hours creating plays.

There were no bedtimes or get-up times or set meals. There was just chaos and noise and laughter. We'd sleep when we were tired and not if we weren't. We'd eat when we were hungry, and we went to school if we wanted to and not if we didn't. The only rules that Mum had were about honesty, integrity and loyalty.

Sonia was a beautiful child, with white-blonde ringlets and darting, interested eyes. In pictures she looks like a street child. I remember her as very feeling, imaginative and passionate.

I had big responsibilities for her. I think we're all defined by our position in the family. Our brother, Richard, was brilliant, as was our sister, Sarah. Then there was me. As the youngest, Sonia had to come in with all guns blazing to find her place. She had great determination and absolute focus and a real concern for others — and that has helped make her into the great producer she is. We would all be talking and no one would shut up. I still remember her saying: "Listen to me."

Mum was absolutely incredible, but our lives were fairly chaotic. And today Sonia's job as a theatre producer is perfect: she can order chaos in a very easy way because she understands it.

By the time she was 11, I was madly in love with my first boyfriend and we older siblings had all left home. Sonia had been one of four children who'd been very close, and then suddenly she was all alone. It must have been really awful. She hated school and got into truancy behaviour. Shortly after that, she went to boarding school, St Christopher's at Letchworth, which took so-called "troublesome" children — which she was. And it was the making of her. In her school holidays I got her jobs in the theatre working backstage, operating the winch and assisting stage management. She was proud of me, but she had this idea that she, Sonia, was running things. She was very intense and determined to be the best winch operator or the best technical operator.

After studying at the Central School for Speech and Drama, she went to work at the National Theatre, where people were impressed but unnerved by her. She had all these ideas that worked. She was in her early twenties, very bright and fabulously good-looking. She had this big mop of hair, and she wore trendy miniskirts. Everybody wanted to go out with her. But she got up some people's noses, and they stopped her getting on in that theatre. Then she met the director Max Stafford-Clark; they got on very well, and he taught her to read scripts and to do a budget. After that she took the plunge and went into commercial theatre — which was incredibly brave.

Men have always adored Sonia, but she spent many years totally focused on her career. Now she's with this lovely guy, Jasper, and has been for a few years. People used to say to me: "Isn't she frightening!" And I'd say: "Who? Sonia? You can't possibly mean my sister!" But she's in a very tough, fast environment, and she has developed a side that is protective of her softness. Occasionally, I'd get glimpses of this fierce, frightening person, and I'd say to her, "You have disappointed me," or: "You have gone too far this time." Poor Sonia.