Jane and Cassandra Austen were the closest of sisters from early childhood. Cassandra was the most important person in Jane's life; Jane looked up to and adored her older sister, who was devoted to her in return. As well as sharing the same education, interests, friends and Christian faith, the inseparable sisters supported each other through various emotional crises and family troubles. Most importantly, Cassandra, who was privy to Jane's imaginary world, supported and encouraged her in her writing.

The Austen Girls explores the lives of the Austen sisters and traces their relationship throughout Jane's life and literary career, until Jane's premature death at the age of forty-one. It also looks at Cassandra's life after the loss of her sister.

`I Jane Austen of the Parish of Chawton do ... give and bequeath to my dearest Sister Cassandra Elizabeth every thing of which I may die possessed, or which may be hereafter due to me... I appoint my said dear Sister the Executrix of this my last Will & Testament.' Jane Austen, 27 April 1817.

The bequest included the manuscripts of Jane's unpublished and unfinished novels.