Regal style of Tara's sister

Who would have thought it? Ten years ago the Palmer-Tomkinson sisters were just the girls who danced with the men who'd danced with the Princess of Wales and skied alongside Prince Charles at Klosters.

Now, unbeknown to each other, they are scheduled to be interviewed, at the same time, in the plumped up, lacquered basement of Blake's hotel in South Kensington.

This strikes Santa (now Santa Sebag-Montefiore) as somewhat risky. "Well she's not sitting here," she says mischievously, "we can't talk about Tara with Tara listening. Anyway she probably won't turn up." Moments later Tara clatters into view, eyes flashing, Marc Jacobs bag snapped under the armpit of her Louis Vuitton coat and Santa rises from our corner to greet her, smoothing down her mid-calf grey skirt and sky-blue cardi. She's tall, blonde, wholesome, poised, the exact opposite of the gipsy-dark, wiry bundle of nervous energy approaching.

"I'm doing my interview," says Tara, "for the Jubilee thing. D'you want to have lunch after?" and rattles off the names of the girlfriends she'll be meeting. You can tell she's wasting her breath; it's like Rasputin extending an invitation to Mary Poppins, and you instinctively know that Santa doesn't do smoky lunches at San Lorenzo. She sets about smoothing out a slight confusion with the photographer, suggesting they repair to her mansion flat "light and airy, big windows overlooking Battersea Park" and generally taking control of the situation with the grace and competence that upperclass girls, before It Girls, were famous for.

Santa, who was named after a barley crop, called Senta, that her father farmed with some success, was known principally for being the older and wiser sister of the reprobate Tara, but now she is a published authoress, whose second 500-plus-page saga, The Butterfly Box, is out this week. It's enough to make your saliva glands go into overdrive to think of the material she has to draw on. There's the Charles, Diana, Camilla triangle, witnessed at close quarters, the Andrew Fergie marriage, likewise.

What about it? "Oh, I couldn't," says Santa earnestly. "I mean, in The Butterfly Box there's a character who takes to drink and he did originally take to drugs but I thought, 'I can't do drugs, people will think it's Tara'."

She claims that Diana never featured much in her life anyway. "We all went skiing one half-term and I have written in my diary, 'Went up the ski-lift with Diana. Full stop.' I look back and I think, 'What did she say?' And it is so frustrating!" These days Santa and her husband, historian Simon Sebag-Montefiore, are frequent guests at Highgrove, but Santa will not be drawn on her royal connections. "I just don't want to appear indiscreet. All I can say is that it is a huge privilege to know him [Prince Charles]. He is a hugely sensitive, caring, generous friend."

So, rather than being drawn from the pistes of Klosters and the bedrooms of Highgrove, Santa's plots are centred in the estancias of the Argentine (Meet me Under the Ombu Tree, her first novel), the Chilean equivalent and Cornwall (The Butterfly Box) and, for her next book (already in the bag) the English quarter of Buenos Aires. Described as an "epic saga of love, possession and metamorphosis" the Butterfly Box is a thoroughly readable tale heady with oldfashioned romance: "I love sensual scenes, not sex scenes. I'm too embarrassed to write that, really," she says dipping her head and laughing. "You only have to read my books to know that I am old-fashioned. I often think that people are born a certain age and I feel I was born in my thirties; all through school I was always far too mature. At 32, finally I think I've grown into my skin."

At boarding school, Sherborne, it became obvious that she and her sister were cut from rather different cloth. "I was the classic schoolgirl success, I suppose. If I had been dying my hair pink and smoking on the roof maybe she would have become vice-head girl. But Tara's always been a wilder character than me anyway. She was always the naughty one and I was just very good I think."

The sisters lived together in London for about a year before she married the man she calls Sebag: "We never saw each other. She came in at 7am after partying all night and I'd be going out to work." When it became clear that Tara's partying was catching up with her, Santa tried to talk to her sister. "I'd confront her when I heard rumours and she'd deny it and I wanted to believe her. Then one weekend, when we were all at home, she came and she said 'I need help'. It was such a relief. We were all just hoping that it was fine and it wasn't."

Santa didn't get involved in family therapy. "My parents went out to Arizona for a family week [with Tara]. She didn't actually ask for me to go out, I would have done if she'd wanted me to. To be honest I think the problems were quite superficial."

It's clear that what saved Santa from the fate of either the sporty Sloane (she played lacrosse for the England juniors) or the aspiring It Girl was meeting her husband. "Before I met Sebag my life was straight out of Enid Blyton. I worked at various cushy jobs (on the shop floor at The General Trading Company, as a PR for Ralph Lauren, to name two) and I was beginning to be asked to do things because of Tara, a Hello! shoot and so on. Then Sebag jumped in" (she gives a To-the-Rescue fanfare). "I think he just said that it wasn't very dignified. And actually I wasn't comfortable doing it. I felt it wasn't me." They've been married for three years now and have an 11-month-old daughter called Lily.

The rather old-fashioned style, the aura of goodness and admission of a certain awkwardness in the company of her husband's sophisticated friends, not to mention the wholesome fresh looks, are vaguely reminiscent of a young, if infinitely more sorted, Diana Spencer. Would she have made a good royal wife? "That's a very good question," Santa blushes. "No, I tell you why I wouldn't, because I hate going out all the time ... I want to be at home with my baby." There is a clatter and Tara roars into view: "I want to be at home with my baby! Which I haven't got yet!" she says ripping off her coat to demonstrate the cunningness of her detachable Wolford cuffs. "No you haven't, but you will," says Santa soothingly and I am suddenly grateful that I never had a golden girl for an older sister.

?The Butterfly Box is published on 7 March (Hodder and Stoughton, £12.99 )