The part about Fanny struck me too. And it gets worse in the seconde part of the letter:

Dearest Fanny must now look upon herself as his prime source of comfort, his dearest friend; as the being who is gradually to supply to him, to the extent that is possible, what he has lost. This consideration will elevate and cheer her.

She is only 15 years old, her mother just died, and she is expected not only to behave like an adult on the occasion, but even to cheer and support her father, as her mother used to do. In short, she is expected to step into the shoes of Lady of the House A.S.A.P.

That such a sensible woman as Jane Austen should expect this tells volumes. She is showing us what was it normal and sensible to expect in those times. Ugh! We are aware now how traumatic this must have been for the poor girl, but apparently at the time, they were not.

Poor, poor Fanny.