“what I meant by the black magic person is the one who is sending the preta.”
I think you may have the wrong picture.
– But let me get the facts straight first. Are there two persons/beings involved in “possessing” your mother?
1. One is your mother’s sister’s husband, and the other is a “preta”?
2. Is your mother’s sister’s husband still alive? If so, does he have the power to send a “preta” to your mother? How?