TILLY: Louise was always a loner. She was not only different from me and our brother, James, but different from anyone I know. She was the classic bookworm. James and I would be playing football and making camps, and Louise would be indoors reading. I get lonely very quickly, and I'm nothing like as confident as I might seem. But Louise doesn't need people, either for company or to reassure her that what she believes is sound. She is one of the few truly independent people I know. She lives in her own head so completely that nothing else exists for her.

The only time I felt her isolation bothered her was as an adolescent. She was an unhappy teenager and a late bloomer physically. From 13 to 15 she had braces, thick glasses and spots, and she combined that geeky look with being bright and bookish. She'd be sitting alone with a book at school break times, which as a teenager seemed desperately sad. I was into my friends, into partying, into school. My sister was on Planet Louise.

I was a bit weird in my own way. I was incredibly driven, a bit anorexic, a real grafter. I've never worked harder than I did for my GCSEs, but Louise's approach was: "Lucky me, I'm clever. I can breeze through life without doing any work." Which is pretty well what she's done.

As a family, we're all quite black and white, but Louise is incredibly opinionated. I find her gaucheness endearing, but she can really put people's backs up. The flip side is that she is incredibly honest, loyal and straightforward. She's also very emotional — a person of huge extremes.

When I became pregnant at 16 I never considered an abortion. Louise puts that down to Catholicism, because she's extremely orthodox, but religion had nothing to do with it. As soon as I knew I was pregnant, I developed an attachment to the child. I felt that I was the mother and I had to protect it. Everyone outside the family was against me — my GP, my teachers; they all thought I should get rid of it. Now I realise that my parents were desperately worried, but they did a good job of hiding it. When I got three As for my A-levels and got into Cambridge, Mum cried, and I realised what a strain it had all been. In a way it helped that I was so young: I assumed I'd manage.

Generally, Louise's life has been rewarding and easy. But she's finding motherhood rewarding but difficult. With small children you're never alone, and Louise needs a few hours a day on her own just to be sane. Before she had children I never saw her as a mother — it wouldn't have occurred to me to ask her for help with Sefi, even when I was struggling. But motherhood has softened her, and we're much closer as a result.

The great thing about Louise is her openness and generosity. If you ring and say, "Can I borrow £500?", she'll say: "I'll send you £2,000 tomorrow." When I had total burnout from my job as a headhunter, she said: "Write a book! My publishers love me and they'll love you!" She does find writing very easy — the words just come. I slogged over my first book, and it took me three months just to think up an idea for the next.

It's a shame Louise's manner does her no favours. She doesn't understand social graces like diplomacy. There are those who think she is slightly mad — our brother, James, always said she had "fans" at Oxford rather than friends — but I think she's just very brave. She's always out there with her extreme views, setting herself up to be shot down. And when she is attacked, she does a very good job of pretending not to be hurt. She appears tough, but she's quite vulnerable, though she'd hate me to say that. Friends think I'm the most feisty, opinionated person they've ever met. But compared to Louise, I'm positively calm and balanced.

LOUISE: If you possess, as I do, a combination of laziness and ambition, being a trash novelist is the perfect job. I don't think I've worked more than four hours a day, tops, since I started 11 years ago. The first thing Tils wrote just wasn't good enough — the tone was all wrong and I had to tell her. She was very upset. But six months later she came back with another novel, which I read with dread, thinking: "Oh no, it's not going to work. How am I going to tell her?" But it was very good. I was really surprised that she could leap from writing badly in quite a literary way to writing pure trash so well.

It's not great fiction we're writing. We write trashy novels which people might find entertaining. In newspaper terms it's like combining the story interest of the Daily Mail with the sex and shopping of the News of the World. I knew Tilly would do it brilliantly. To date, her life has been one seamless list of achievements.

We had a disgustingly idyllic childhood. We were comfortably off, and it didn't hurt that we were both clever and got full scholarships to our schools. Tilly was the rebel; even now she won't be pigeonholed. She votes Conservative, yet she's a republican. She's never had respect for authority. She was 16 when she got pregnant.

I remember walking into my rooms at Oxford and finding her there — she'd come to ask my advice and must have climbed through the window. It never crossed my mind that she'd consider an abortion. We're Catholics, and killing your baby means excommunication. It wasn't an option. My parents were fantastic. My father said: "Darling, how wonderful, I'm going to be a grandfather." Their attitude was that Tilly could handle it. So she had the baby and went up to Cambridge with Sefi in tow, which is the kind of girl she is. But I don't remember the problems. She's reminded me that her Catholic school expelled her and only let her come back to sit her A-levels.

And she was escorted to and from the exam room to spare the other girls from moral contamination. It's amazing, given this, that she got into Cambridge.

Cambridge gave Tilly a house and organised daycare for the baby and a schedule she could work to. We've got lots of pictures of Sefi climbing up the steps of St John's College in her nappy. Her house was always full of friends but there must have been moments of exhaustion and loneliness. She realised, playing pat-a-cake at five one morning, that she was on her own there. Her friends would be coming home from all-night sessions and she'd just be getting up with the baby.

But my sister is one of the few people in the world who could handle all this. Her independence combined with a kind of fierce bravery got her through. Okay, I'm sure she had tough times. But I think the hardest time came later. Her first job didn't pay very much. She was living in London with a nanny, hating her job in recruitment. I was going through my own unpleasant time, working as a music PR. I didn't enjoy my job and my employers didn't like me. But I suppose, compared to Tilly's life, it was nothing. She did say she was exhausted and stressed, but she didn't ask for help, and it's never hit me until this moment that I didn't offer. You really don't have any idea how hard having a baby is until you've done it yourself.

Maybe she felt she had to work twice as hard to prove herself, because within a few years she was the youngest partner, earning around £300,000 a year. But headhunting is very stressful, and once she'd saved enough money she quit. She bought a cottage near my parents and took a teacher-training course. She loved the concept but, being Tilly, she couldn't do all the jumping through hoops. Then I think she noticed that my life as a novelist is pretty jammy. You get paid incredibly well and get loads of time off, so it fits perfectly with children.

Tils's husband is a financier. He's a very successful, very rich man. They've bought a house in Bel Air and one in Kensington, but Tilly doesn't feel she can let her own career slacken off. She doesn't have anything to prove — she's done that already. But sitting around while a man keeps us is not our style.