Rabbi Silverman: The truth about my sister Sarah

Meanwhile Sarah Silverman, 37, has described her sister as "super-duper crazy Reform" and regularly mentions with glee in interviews the fact that Susan is a rabbi.

"Sometimes I'll be working in the cheder ochel [dining room]," says Susan. "I'm covered in gloop, it's late at night, and I'm sweeping the floor, and I think, ‘I wonder what Sarah is doing right now?'"

One of four sisters - Jody and Laura are both Hollywood screenplay writers - Susan says: "We were really liberal, and so our parents talked a lot about progressive issues like racism and sexism. We were always the only Democrats around because we were the only Jews around."

Their father, Don Silverman, trained as a social worker but went on to run a retail business. He and his wife Beth, the director of a community theatre, were heavily involved in the Democratic presidential election campaign of George McGovern in 1972.

"It was a huge part of our lives," says Rabbi Susan. "These values were really central to all of us. When I was finishing college and I met my then boyfriend, now husband, Yosef, through the [anti-apartheid] divestment movement at university, I realised that all those values that I had, had a Jewish source. So I became interested in Judaism and went to rabbinic school."

Susan is not surprised that her sister wound up as a comedian. She says Sarah always traded on her signature combination of cutesy and shocking.

"She was always adorable and funny," says Susan. "One of my closest friends has a favourite memory of her from when we were at college at Boston University. I was 18 and she was 11 or 12. I took Sarah with me on spring break to Florida. We were on the beach, and she pulled out a cigar and started smoking it. It was completely unexpected. At 11 or 12 she was tiny - she looked like she was eight or nine. It was hilarious. She was always vying for attention. The funniest story was that when our parents told us they were divorcing we were all crying. Being the youngest, we all gathered round Sarah and said, ‘It will be OK,' and she said, ‘No. I'm crying because I'm dancing and nobody's watching me!'"

Susan also remembers how Sarah first learned how to make people laugh, aged two.

"First of all, she was just so cute ... and really hairy for a little kid," she says. "We always used to say, ‘Oh sis, you have lovely arms and legs,' because we were afraid that people will make fun of her. Her father, my dad, used to hold her on his lap when she was two, and she had these big brown eyes and this hair cut at ear's length and she looked like a little monkey. And he would make her say: ‘Bitch! Bastard! Damn! Shit!' and people would be laughing hysterically.

"My father had a funny sense of humour and it just really worked. We were so delighted by it because it was just so funny. My grandmother would be mock horrified, but Sarah was just so charming it was funny."

But she does not feel jealous of her sister's success. "She's seven years younger, so I always felt very loving and protective of her," insists Susan. She says that she has been happiest when she and her three sisters have come together for family occasions and lie on her mother's bed making each other laugh.

"And I really bathed in it. I love all the attention. With all my sisters I feel I love the fact that they shine. It makes me feel so happy."

Sarah has been dating non-Jewish American comedian Jimmy Kimmel for the last five years. They recently split up and got back together. But while Susan would have preferred her sister to be with someone of the faith, she would still be happy to welcome him into their large, loving family.

"I just want Sarah to do what's right for Sarah," she says magnanimously. "We would love for my kids to have cousins. I like to know that Jews are raising Jewish families, but it's not for me to decide for other people, and I love Jimmy and I'd be very happy for him to be in my family. He's a lovely person. We all love him."