Sarah Sands: I'm not losing a son, but gaining two friends

Unlike Tolstoy, Joanna Trollope believes that every unhappy family is unhappy for a common reason. The fault line is in the structure of the institution. Her new book, Daughters-in-Law, examines a mother's anguished yielding of her sons to their wives. Rachel is a professional cook, her husband a bird painter, and they live in a ramshackle house in Suffolk. Picture the idyllic scene. Rachel has been at the centre of her world, her kitchen always full of her sons and their friends. She cannot understand why their wives resist being subsumed into her embrace. As the newest and most troublesome wife tells her husband: "Now we're married we don't belong to our parents the way we did before."

I read this book with some trepidation, being a fairly new mother-in-law. I have a shrine of wedding photographs, including one of the overwhelmingly pretty bride and groom leaving the church, and of the groom clutched by his beaming mother. I see how that might send shivers through a new wife.

As a genre, mothers-in-law enjoy slightly less popularity than Middle Eastern dictators. One of the best television performances of the year has been Francesca Annis playing an Aga heroine in The Little House. She offers to buy her son and daughter-in-law a cottage at the end of her drive, and to help to look after her first grandchild. Naturally, this is a horror story. The daughter-in-law ends up murdering the mother-in-law, to universal audience approval.

Not realising quite how this pastoral tale was going to end, I urged my own daughter-in-law to watch the drama, as a bonding experience. She was alone in her flat at the time, and terrified. Her friends wondered afterwards what I was up to.

There have been other MIL crimes. I passed on an Alberta Ferretti slate blue suit, which I thought would suit my daughter-in-law's gorgeous Scandinavian colouring. She dutifully wore it to the office, to peer bafflement. It was so unlike her own distinctive style. And why would you want to wear your mother-in-law's clothes? I have suggested too many arrangements, and I am always reluctant to see them leave. The one music-hall quality I lack is fault-finding. I regard my daughter-in-law with unconditional awe.

My enlightened feminist friends with younger sons still shriek at the prospect of the transfer of primary love and loyalty from mother to wife. A mother with an adorably blond and stocky toddler told me she fervently wished he would be gay. This is the strength of passion that future daughters-in-law face.

All adult human relationships require restraint and kindness to survive, but it is especially true of the mother-in-law/daughter-in-law axis. Mothers-in-law can make invitations sound uniquely oppressive. No wife enjoys picking up the phone to find her mother-in-law looking for her son. Thankfully, email has saved daughter-in-laws from reproachful, inquisitive or demanding telephone conversations.

The rule is never to offer unsought advice or use possessive knowledge of your son. It is curious to cuff a boy round the ear one day and treat him with distant respect the next, but the relationship has to alter. In Trollope, the burden of a mother's love is lifted when she turns to her work and the solace of her husband. I have discovered a further joy, which is the adult and equal friendship of a son and his wife.

Sarah Sands is deputy editor of the 'London Evening Standard'