The Youngest Miss Ward — Joan Aiken

Harriet Ward, known as Hatty by her sisters Lady Bertram and Mrs. Norris, does not at the age of twelve seem destined for a career of infamy. However, treated with utter contempt by her elder sisters, she is banished by her father to an uncle's establishment in Portsmouth where she must do her best to fit in with troublesome male cousins and their sickly sisters.
But she has not seen the last of her frightful family. Indeed, events contrive to bring back into her life the haughty Lady Ursula, a friend of her father's and a main source of her grief at home. But why should this forbidding woman be travelling so far to attend the funeral of a distant cousin?

Rumours run riot and, in an unexpected twist of fortune, Hatty soon finds herself in competition with Lady Ursula for the attentions of a certain Lord. Two more startling turns of the wheel leave Hatty in a position where she will never again be mentioned by the Mansfield Park set.