Martha Wainwright was keeping it in the family at the Union Chapel in Islington last week. Arcangelo, the singer-songwriter’s three-year-old son, joined her on stage and had the audience eating out of the palm of his tiny hand; the spectral presence of her mother, the folk legend Kate McGarrigle, was never far away; and the evening was peppered with references to intense sibling rivalry with her irritatingly talented brother Rufus.