what might have modivate harriet's mistress to bequeathing harriet as a slave to her five-year-old neice?

Harriet quite liked her mother's mistress but upon her death ownership of Harriet was transferred to the mistress' niece. I think the mistress was hoping the company of a child might forge a caring relationship between the two. The niece,however, was only three years old thus Harriet's actual master was the father, Dr. James Norcom. This man was not a nice person.