Excerpted from LEARNING TO READ by Frederick Douglass
I lived in Master Hugh's family about seven years. During this time, I succeeded in learning to read and write. In accomplishing this, I was compelled to resort to various stratagems. I had no regular teacher. My mistress, who had kindly commenced to instruct me, had, in compliance with the advice and direction of her husband, not only ceased to instruct, but had set her face against my being instructed by anyone else. It is due, however, to my mistress to say of her, that she did not adopt this course of treatment immediately. She at first lacked the depravity indispensable to shutting me up in mental darkness. It was at least necessary for her to have some training in the exercise of irresponsible power, to make her equal to the task of treating me as though I were a brute.
What does the bolded sentence mean? I can't understand its structure, It is due to my mistress to say of her?, it's odd.