In the story which is told by Doctor Manette in the letter who is the girl in the room who says My father My brother My husband 12345678910 Hush!!
The girl and her brothers never give their family name. The doctor addresses each of them by "the girl," or "younger brother," ect..
But is it her elder sister or younger?!?
They talk about a younger sister later in the novel. So I'll have to go with the younger.
The girl in the room was eldest sister (something in twenties), the boy or the kid you refer too is the middle one and the youngest sister that was hidden away by the boy, as it later turned out, was Madame Defarge.
Source: Book the Third, Chapter 12, "Darkness".
