Feuds between sisters are well enough known: movie stars Joan Fontaine and Olivia de Havilland maintained a sisterly quarrel all their lives. The writers Margaret Drabble and her sister A S (Antonia) Byatt only meet - acknowledging each other with a formal nod - at funerals, never having patched up a family difference. Among the notorious Mitford sisters, Jessica was a Communist and Diana was a Fascist (and the mother of Desmond Guinness, who saved Georgian Dublin from destruction) - throughout their adult lives they were not "on speakers".
But if there is sisterly feuding, there are also strong bonds between sibling sisters. The recent film A Quiet Passion featured the American poet Emily Dickinson and her sister Lavinia; had Lavinia not obsessively curated her sister's poems after Emily's death, her work would have been lost to posterity. (And if one Muriel hadn't squirrelled away her sister's letters, we would know less about the early life of Margaret Thatcher and her feminine penchant for pretty lingerie.)
Jackie Kennedy and her younger sister Lee Radziwill seemed to have had that sisterly bond which was both competitive and supportive. Both were urged to "marry well" by their ambitious mother, and indeed both did - Lee got her prince and Jacqueline her president. They had depended on each other as young girls during their parents' divorce - which had not gone well and had attracted much gossip - and that emotional link seems to have remained.
Constance Markievicz and her sister Eva Gore-Booth are forever fixed in poetic image by Yeats' elegy for the Sligo sisters: "The light of evening, Lissadell/ Great windows open to the south/ Two girls in silk kimonos, both/ Beautiful, one a gazelle." The poet draws some harsh conclusions about what the passage of the years may bring - yet the sisters are remembered as bound in an immortal coil.
Constance and Eva remained as close as siblings can be - Constance's most compelling letters from prison are addressed to Eva (who lived with her female partner, Esther Roper), beginning "Dearest Old Darling", while Eva dedicated many poems and illuminated texts to Constance. They had a harmony of soul. They had been only two years apart in childhood and were only one year apart in death.
Constance and Eva were political in different ways (Constance the activist, Eva the pacifist).
Virginia Woolf and her sister Vanessa Bell were artistic in different ways - Virginia the writer and Vanessa the painter. Virginia became the more famous, but Vanessa was, and is, esteemed as an artist. Virginia seems to have depended on her older sister, emotionally, and constantly refers to her in her diaries. Virginia was childless, but greatly cherished Vanessa's children, and Quentin Bell, Vanessa's son, became Virginia's most authoritative biographer.
A poignant story of sisterly attachment is that of Edith and Rosa Stein, who died together at Birkenau concentration camp in August 1942. Edith Stein was a brilliant philosopher from a Jewish family - she pioneered studies on the quality of empathy - who became a Carmelite nun, as Teresa Benedicta of the Cross.
The Nazi authorities started raiding monastic communities for anyone of Jewish origin and she moved to the Netherlands, along with her sister Rosa. But they were found, taken to Auschwitz and then to Birkenau. Rosa never left Edith's side, although had they split up, they might have had a better chance of survival.
This week, a TV documentary on the history channel Yesterday featured three sets of sisters - the Mitfords, the Bouvier girls, Jackie and Lee, and the aviatrix Amelia Earhart and her younger sister Muriel (who provided Amelia with some of the money to buy her first plane).
As it happens, I've also written a one-hour drama about two sisters, Eva Braun and her younger sister Margarethe, known as Gretl, to be broadcast on RTÉ Radio One.
Eva and Gretl were also inseparably close, as children and as young women - they shared a house together in Munich. When Hitler was at the height of his powers (and it must be admitted, electoral popularity), they both lived the high life, and Gretl became part of the inner circle at the Bavarian Alpine retreat, Berchtesgaden.
Neither of these young women were, in themselves, bad people. Neither were they stupid - Eva Braun was an accomplished photographer when photography required more technical knowledge than it does today. But they were selfish and heedless.
They chose to turn a blind eye to the evil around them.
They chose not to see what was happening before their eyes - and they reaped the consequences.
Eva, of course, chose suicide at the age of 33, while Gretl's subsequent life was dogged by humiliation and more family suicide.
Sisters can be competitive or supportive, and groups of singing sisters are also a long tradition (The Andrews Sisters, The Beverley Sisters, The Nolans). In White Christmas, Rosemary Clooney and Vera-Ellen perform the classic Sisters song, which tells us much about the sibling bond: "God help the mister who comes between me and my sister / But God help the sister who comes between me and my man!"