While famous English novelist Jane Austen is best known for skewering the vagaries of romantic love in her novels, there is another, equally complicated and meaningful type of relationship that runs through them, too—that of siblings, reports CNN.

Sisters, in particular, influence the thoughts, opinions and actions of many of Austen’s heroines, acting as confidantes or foils.

Sisterhood defined Austen’s own life, too, as underlined by a long, chatty letter she wrote to her elder sister, Cassandra, which is expected to fetch up to $400,000 at a Sotheby’s auction in New York in October.

Nearly three years apart in age, the two sisters were extraordinarily close; so close that their mother once said, “if Cassandra’s head had been going to be cut off, Jane would have hers cut off too.”

As Kalika Sands, Sotheby’s Head of Books and Manuscripts New York, notes, they had an “incredibly close emotional bond” as well as a “very strong intellectual bond.”

This relationship is evident in the letters they wrote to each other. In this one, Austen rattles through an account of her days in Bath, gossips about their neighbors and relatives, shares news about their friends, pokes fun at Cassandra’s ill health, and laments the family’s diminished social status.

Neither of the Austen sisters ever married and they remained each other’s closest confidante until Jane’s death at age 41.

It was Cassandra who painted the only surviving portrait of Austen during her lifetime and who destroyed many of her sister’s letters following her death.

Although this has rendered Austen a somewhat elusive historical figure, “if we take a big step back, it’s actually an incredibly generous gesture of a sister protecting a legacy,” Sands said.