Is this parentification?

Austen's niece Fanny look over the "lady of the house" role when her mother died and didn't marry until she was 27 - into another established family

This was common at the time - someone had to step up to play mother / become the lady of the house. Fanny Knight (Austen's niece) did it. This would be considered parentification now. (Definition "the assumption of a parentlike (or adult) role by a child.")