What might have motivated Harriet's mistress to bequeathing Harriet as a slave to her five year old niece?

In context, we might infer that her mistress intended for Harriet to be a caregiver for the five-year-old, Emily Flint. Based upon the fact that her mistress was genuinely fond of Harriet, and had even taught her to read and write, the woman must have believed that this would be a good idea.... she was wrong. The Flints were cruel and callous.