She was the daughter of William Stephen Poyntz, a Liberal MP, and his wife, the former Hon.

Another sister, Frances Selina Isabella Poyntz, married Robert Cotton St. John Trefusis, 18th Baron Clinton, and, on his death, married Sir Horace Seymour, MP.

[3] The Poyntz family had homes at Midgham House in Berkshire and Cowdray Park in West Sussex.

[4] Their children were: The 2nd Marquess died in 1867 and his estates passed to the oldest son William.

Isabella moved to 32 Upper Brook Street, Mayfair, where she died in 1879, aged 76:[6] she was buried with her husband in the Cecil family chapel at St Martin's Church, Stamford.