Mothering both camp pop-crooner Rufus Wainwright and his equally talented sister Martha (both playing in Brighton next month) should be enough to get any woman a lifetime's achievement award for services to music.

But Kate McGarrigle and her sister Anna hold their own cult status, writing songs of exquisite heartbreak and harmony which, since the Sixties, have seen them passionately loved by the folk scene and beyond.

Born in Montreal with a mixed English and French-Canadian background, the McGarrigles grew up in the Laurentian Mountains, Quebec. Taking piano lessons from the village nuns, they also learned to play the banjo and accordion which lay around the house, while family singing sessions around the living room piano were a regular habit.

By the time they were teenagers they had absorbed a rich musical heritage from Victorian ballads to country blues and though Anna went on to study painting and Kate engineering, they were, simulataneously, becoming stars of Montreal's folk scene.

When their songs began finding their way into others' repertoires (the famous Heart Like A Wheel, Anna's earliest stab at songwriting, became the title track on Linda Ronstadt's first number one), Warner offered them a deal. And the sisters responded with their 1976 eponymous debut, a uniquelymoving blend of home-grown folk and striking vocal harmonies which became Melody Maker's album of the year and caused a Rolling Stone reviewer, surely for the first time, to use the compliment "delightful".

Almost three decades on and the McGarrigles (who still look and sound eerily identical) have amassed a string of acclaimed albums and an impressive list of collaborators which includes Lou Reed, Emmylou Harris and Nick Cave.

One of the most popular has been 1998's The McGarrigle Hour, a sort of musical knees-up with family and friends (including Kate's ex-husband Loudon Wainwright III) on which the repertoire ranged from the rollicking Sixties drug anthem Green Green Rocky Road to the high society complaint of Cole Porter's Allez.

But the sisters have never sounded stronger that is, tenderer than on current album La Vache Qui Pleure, an all-French record tinged with melancholy sweetness and intelligent grace on which subject matter ranges from the title tale of a cow mourning her calves to the signature wit of Petite Annonce Amoureuse, in which a woman advertises for a 5ft 3in man but settles for a lover who is 5ft 2in.

Though Montreal still gets the pick of their concerts, the McGarrigles are no strangers to these shores and have often been spotted accompanying Rufus and Martha (they tend to ask for stools these days but can still out-harmonise and out-sit their young family members). This, however, is a rare chance to see folk's foremost siblings headline in the UK.