Who is Cassandra Austen from Jane Austen's Letters and what is their importance?

Cassandra Austen was born on January 9th, 1767. She was George and Cassandra Austen's first daughter. Cassandra Austen was educated with her sister at the Abbey School from 1785 until 1786, she was Jane's only sister and best friend. The two girls were very close, and they spent much of their time together. Most of Jane's correspondence is directed to Cassandra Austen while one of them was visiting. Cassandra Austen was the executrix of Jane Austen's will and Jane Austen's heiress. Cassandra Austen died unmarried on March 22, 1845.