The famous Fiennes

'I actually think Magnus is a genius," says Martha Fiennes, of her younger brother. "I really do, I've said it to other people and they've said 'Yeah, I think he is'. I'm chucking stuff out barely finished, but Magnus is picking up on it; I think he tunes into a frequency. What Magnus has done is so completely brilliant. Handel wrote The Messiah in 12 days, I understand, and Magnus has done exactly the same."

She is talking about the soundtrack that Magnus has created for her first digital installation, Nativity, on display for the Christmas season in a specially constructed chalet in Covent Garden piazza, in London. In fact, Handel is thought to have spent 24 days on his oratorio, and the comparison with her brother's composition is a little ridiculous. But then good big sisters are supposed to be generous and indulgent, and perhaps this unembarrassed eagerness to talk up each other's talent is one of the advantages of working with family.

This is something in which the Fiennes siblings specialise: of the six, five work regularly in the film industry, the eldest brother Ralph being by far the most successful. He made the leap from a stage career to Hollywood with Schindler's List in 1993, and was followed by Joseph and Shakespeare in Love in 1998.

Meanwhile, Martha was directing commercials and music videos, Magnus was working as a pop producer and Sophie was serving an apprenticeship with the film director Peter Greenaway and managing a dance company. Since then, Martha has directed two films of her own – Onegin and Chromophobia – both starring Ralph. Magnus has composed numerous film and television scores, including those for Martha's films, and worked with topline acts such as Pulp, Tom Jones and Marianne Faithfull, while Sophie has become a director of arts documentaries. Their other brother, Jake, Joseph's twin, is a gamekeeper, while their foster brother, Michael Emery, is an archaeologist.

"My mother was amazing at encouraging us," says Magnus, joining Martha and me at the bar where we are meeting before an arts function. "Her mantra was whatever you wanted to do, whether you wanted to be a dustman or a zookeeper, that was absolutely brilliant as long as you did it to the best of your abilities. We're not a very academic family, which is not to say we're not all very well read but none of us went to university. That probably gave us a bit of edge and a punk-rock approach to ideas."

There is a clear family resemblance, but Magnus would not be mistaken for his famous brothers. In a skinny black suit, he combines angular Englishness with a languid manner perhaps picked up in Los Angeles, where he sometimes lives. Martha is glamorous, with long hair and striking jewellery. Separated from her long-term partner and father of her children, George Tiffin, she plans to marry Issam Kabbani, a hedge-fund manager, next year.

Their upbringing was unusual, with a strong flavour of 1960s Chelsea bohemia transplanted to the country. Their father, Mark Fiennes, was the eldest son of an industrialist knighted by Harold Wilson. He was a distant cousin of the Prince of Wales, as well as the explorer Sir Ranulph Fiennes, and became a farmer in Suffolk. But after he married novelist (and later painter) Jennifer Lash, known to all as Jini, he followed her into the arts, developing a passion for photography, working for magazines and recording the restoration of Windsor castle.

They moved between East Anglia, Wiltshire, London and the west of Ireland, living in big old houses that they would do up and sell on. In Ireland they set up a postcard business, travelling around taking pictures and spending hours in the dark room. While Martha says she never wanted for anything, and was used to being the new girl at school, the family finances were uneven, and there was a strong pressure for hard work. "People would come and help out, Jini had a tough time managing on her own," Magnus says. "She had six children under seven and our father was trying to career-change – it was chaos."

"But the house was tidy, not everything strewn everywhere," Martha chips in, protectively. Jini died in her 50s in 1993, from breast cancer, just as Ralph's career was taking off so she never saw his success, or knew her grandchildren. Mark died a decade later, so the children have lost both parents.

Sophie recently described their upbringing as a kind of "creative experiment", and Magnus and Martha agree that their mother was complicated. Martha remembers her reduced to weeping despair by bad reviews. Lash is not much read nowadays but was remarkably successful in instilling her love of the arts in those she loved.

"There was always analysis, be it homework or a picture," Magnus says. "Anything that was created, there was immediately a discussion about it. Could it be this? Or should we try this or that? That was very important, don't you think? Never 'That's lovely, darling, let's put it on the shelf.'"

"We always engaged in exhibitionism and performed in an ad hoc way," he says when I ask if there were living-room productions starring Ralph and Joe, and describes a version of charades that evolved into a series of plays around multi-syllable words. Although she caught on to pop culture in the 1970s, Jini disapproved of TV and one of Martha's favourite stories is of Sophie once asking neighbours if they had been watching their "bourgeois television", having picked up the expression from sniffy grown-ups. Magnus recalls a record collection that was mainly classical, with the New Seekers, Rolf Harris and James Taylor thrown in.

They laugh off a question about whether they had to compete for parental attention, and reminisce about hours spent washing up. Years of being on their own in this privileged version of benign neglect bound them tight. Martha and Magnus now have children of similar ages and meet regularly.

"Luckily, no one else is in music; I'm sure I would feel competitive if they were. That's probably why I'm not in acting," is as close as Magnus will come to admitting any awkwardness. "All of us like to ask each other's opinions – lots of opinionated opinions. But we only feel like a gang when we're out together."

They are sometimes described as a Catholic family because of Lash's background. Martha refers to their mother's Catholic guilt, and Magnus to a Protestant work ethic passed down by their father. Christmas was a special time with its own traditions, but Magnus says they went to Midnight Mass "mainly for a good old sing", and neither he nor Martha claims any strong faith or religious reasons behind their digital Nativity, in which the traditional elements – Mary, Joseph, baby, wise men, angels, a dog – interact against a changing background in a random way determined by computer coding.

The launch at the Freemasons Hall in London last month was the first time Magnus saw the work, having composed his score in California based on a series of "frantic Skype sessions". He says that ideally the music would be randomly generated in the same way as the images, but he has only created about 90 minutes of sound, which is played on a loop. "One can approach the piece in an entirely secular way," he says. "For me, here is an event that's about rebirth, the birth of possibility and the birth of hope, and that in itself is an incredibly powerful thing. It's a story of mythological importance."

Martha's clearest enthusiasm during our conversation is for her brother's ability to divine her intentions. "I think we immediately know what lies at the heart of an idea," says Magnus.

A film Martha hopes to make soon, about a child's torso found in the Thames in 2001, is a collaboration with her brother Joe. Two of her children have already been in films with Uncle Ralph: Hero as the young Voldemort in Harry Potter, Mercy with Keira Knightley in The Duchess. So far, four is the maximum number of Fienneses to have worked on one project. Maybe one day all the siblings will work together – or perhaps more, if the next generation joins the business.