When I started research for my novel involving Jane Austen, I was most interested in learning -- along with details of everyday living -- about Jane Austen's siblings, who loom particularly large in her life.
Cassandra, of course, her lifelong companion, perhaps the most influential but also the most mysterious.
My personal favorite -- he may have also been JA's favorite -- is the charming and mercurial Henry, whose residence in London is important to JA's writing career, since he is the one she stays with when she goes to town to deal with publishing matters. He was also instrumental in dealing with those publishers. He was the only one of the Austens to live in London (although Edward went there from time to time, like every proper wealthy gentleman) and therefore it is to him we owe JA's experiences and impressions of London, which add so much to her novels despite their largely pastoral setting.
James, the oldest brother and the clergyman, was considered the writer of the family when they were all growing up, and is thought to have encouraged JA's writing. The vexed relationship with his second wife (everyone loved Mary Lloyd until he married her, and then she seems to have gone out of her way to annoy them all) seems to have also provided JA with some close-up insight into difficult people.
Edward, by virtue of being richer than anyone else, provided the raw material for JA's accounts of life in homes like Mansfield Park and Kellynch Hall, something she would not have had otherwise. And of course, from 1809 he provided JA and her sister and mother with a place to live, the stability of which appeared to fuel JA's creativity and productivity.
The sailor brothers, Francis and Charles, though away for long periods of time, were important and very well loved. They bring a dash of sea air and a flavor of the world outside the small towns where JA's life (and her novels) for the most part were set. Persuasion in particular but also Mansfield Park rely heavily on the details of naval officers' lives ashore.
But four of those siblings -- James, Edward, Francis and Charles -- also had children, and toward the end of her short life JA was starting to take particular interest in several of them, as I realize now, getting to the part of my novel that is set in Chawton and Alton in 1816 and 1817 and rereading the letters from that time.
Fanny, oldest daughter of Edward, (that's her likeness above, done by Cassandra) apparently had a special place in JA's heart, though from the evidence it is hard to say why. Around 1814-1815 she was writing her aunt seeking advice about her romantic life, which was as complicated as anything in an Austen novel and yielded a series of intense, very funny replies. But it is one of the very last letters JA wrote to her, in February 1817, that is my absolute favorite:
You are inimitable, irresistable. You are the delight of my life. Such Letters, such entertaining Letters as you have lately sent! Such a description of your queer little heart! Such a lovely display of what imagination does! You are worth your weight in Gold, or even the new Silver Coinage. I cannot express to you what I felt in reading your history of Yourself, how full of Pity and Concern and Amusement I have been. You are the Paragon of all that is Silly and Sensible, common-place and eccentric, Sad & Lively, Provoking & Interesting. -- Who can keep pace with the fluctuations of your Fancy, the Capprizios of your Taste, the Contradictions of your Feelings? You are so odd!-- & all the time, so perfectly natural -- so peculiar in yourself & yet so like everybody else! It is very, very gratifying to me to know you so intimately. You can hardly think what a pleasure it is to me to have such thorough pictures of your Heart. Oh! What a loss it will be, when you are married. You are too agreeable in your single state, as a Niece. I shall hate you when your delicious play of Mind is all settled down into conjugal & maternal affection....
JA did not live to see this happen. Fanny was married in 1820, to Sir Edward Knatchbull, a widower 12 years older than her who already had six children, bore him nine more, and lived to 1882, becoming very rich and grand, something of a snob, and in her last years, tragically senile and thus unable to contribute to the memoir that other members of that generation worked on.
Anna, oldest daughter of James by his first wife, and the same age as Fanny (both born 1793) was another favorite of JA's. At one point around 1814 she was working quite seriously on a novel, which she shared with Jane and Cassandra and their mother. JA's letters commenting on this work to her remain the clearest accounts we have of JA's own views about writing and are extremely interesting to scholar and layman alike for this reason. She is both encouraging and critical, always kind. (Though letters to Fanny sometimes contain catty comments about Anna, there are never any catty comments about Fanny to anyone, in any letters.) Anna, after an even more bumpy romantic career than Fanny, married a clergyman in November 1814 and started rapidly producing children ("Poor Animal, she will be worn out before she is 30 --I am very sorry for her," JA observes to Fanny in March 1817, when Anna is pregnant with her third child.) Anna seems to have been smarter than Fanny, but more difficult.
James-Edward, James's second child and only son (born 1798), would, near the end of his own long life, write the first biography/memoir of JA, with the help of Anna and his younger sister, Caroline. He graduated from Winchester College (actually a high school) at the end of 1816 and looms increasingly large in her letters after that. He was also writing a novel and seeking his aunt's advice.
"He grows still, & still improves in appearance, at least in the estimation of his Aunts, who love him better & better, as they see the sweet temper & warm affections of the Boy confirmed in the young Man," JA wrote her friend Alethea Bigg in January 1817.
Caroline, his younger sister, born 1805, seems hardly old enough to remember JA, yet she provided some of the most vivid memories for her brother's book. She also carried on a lively correspondence with her Aunt Jane, starting about 1815, and was the recipient of the famous backward lettter of January 1817.