A Celebration of Kate McGarrigle, Royal Festival Hall

This is very much a family affair. Which is only fitting for a celebration of Kate McGarrigle, somewhat of a mother to the Canadian folk scene, and literally mother to two extremely talented singer-songwriters, Martha and Rufus Wainwright. McGarrigle died in January, aged 63, and the concert was organised by her old friend Richard Thompson, currently curating the Meltdown festival at the Southbank Centre.

There are McGarrigle's sisters, Jane and Anna, as well as the latter's husband and children here. Both Wainwrights are in attendance. Thompson's former wife, Linda, and their kids Teddy and Kami, lend their voices to a chorus that seems to sing of talent running in families. Add to this self-proclaimed "clan" such musical mates as Nick Cave, Lisa Hannigan, Emmylou Harris, Jenni Muldaur... I could go on, but suffice it to say, it's a pretty magical melting pot.

The scale of the three-hour event and its collaborative nature means things are a little shambolic, with a fair amount of stop-and-start. It's mostly McGarrigle's music, and Rufus says, of playing his dead mother's songs: "It's very difficult, on the one hand; but extremely uplifting on another – so let's continue the ride". He does so, his theatrical rendition of "Southern Boys" backed with cooing vocals from Anna McGarrigle and Muldaur.

Teddy Thompson's pared-down performance of "Saratoga Summer Song" provides one of the first breathtaking vocal displays of the night. It's a beautiful and heartfelt, but terribly fragile, performance and it makes me nostalgic for a summer I never even experienced.

The second half opens with a home video of McGarrigle jokingly lip-synching along to one of her songs on the radio. It's innocently intimate footage, and sets the scene for a far more emotionally raw second half.

"Mother Mother" is an electric number, with Cave's voice creating a sense of menace, complemented by Martha's whispering vocals, and crashing drums from Martyn Barker. Neil Tennant of the Pet Shop Boys sings "I Cried For Us", but with sweeping strings and his falsetto voice, this love-lorn lament becomes a little too syrupy. "I Eat Dinner", by contrast, manages to stay on just the right side of weepy – Rufus and Harris's voices complement each other as they sing of eating leftovers at the kitchen table, and the loss of candlelit romance.

"Proserpina" – the last song McGarrigle wrote – is led by Martha. But singing her mum's song about motherhood becomes too much; she chokes, can't sing. But her yearning gestures speak volumes, and there are few dry eyes offstage either.

"Life is short and sweet/ This much I know," sings Krystle Warren, aptly; another revelation, her Nina Simone-esque voice is astonishingly powerful. And McGarrigle's fans weren't just fellow folkies; the novelist Michael Ondaatje gives a reading from a novel in which he quoted McGarrigle's "Mendocino": "Never had the blues from whence I came/But in New York state I caught 'em".

Rufus performs his own number about her, "The Walking Song". Despite saying "Let's consider this as a chill-out moment; it's been so emotional," he ends with a "thanks mom" and an audible sniff. He – and the rest of the clan – are met with a standing ovation. Emotional stuff indeed.