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

In the autobiographical Incidents in the Life of a Slave Girl, Harriet Jacobs gives us an account of her years living as a slave. Linda Brent, a pseudonym that Jacobs used, faces many hardships while under the thumb of slavery, some that almost seem almost too terrible to be true, but as she states in the preface to the book:

I am aware that some of my adventures may seem incredible; but they are, nevertheless, strictly true. I have not exaggerated the wrongs inflicted by Slavery; on the contrary, my descriptions fall far short of the facts.

The first mistress that young Linda had was a woman who promised Linda's dying mother that "her children should never suffer for any thing." And so, when this mistress "sickened and died" when Linda was twelve, Linda and her loved ones hoped that the woman had made some sort of provision for Linda to be set free. Unfortunately, as it turns out, the mistress wrote in her will that Linda should be given to her niece, who was only five years old at the time. In fact, all of her slaves were "distributed among her relatives," rather than given their freedom.
The reasoning behind this which makes the most sense is that while this unnamed mistress was kind to Linda, she was still a white woman who had racism ingrained in her from her upbringing. Because of this, she saw Linda as a slave and a commodity, rather than as a person. This answer is heavily enforced by two quotes from the end of the first chapter. The first quote comes from Linda reflecting on her relationship with this mistress:

My mistress had taught me the precepts of God's Word:"Thou shalt love thy neighbor as thyself." "Whatsoever ye would that men should do unto you, do ye even so unto them." But I was her slave, and I suppose she did not recognize me as her neighbor.

The second quote is very last sentence of the first chapter, and demonstrates that Linda knows what most white masters thought of their slaves:

These God-breathing machines are no more, in the sight of their masters, than the cotton they plant, or the horses they tend.