I reread the thread to get an idea of the problem. The following are my observations.
1. Your suspicions about the “black magic person” may be correct. I have heard of such instances in Sri Lanka. Abhaya Thero could be helpful if you could contact him. I will see if any of my contacts know how to contact him. He used to investigate such cases in Sri Lanka before becoming a bhikkhu. The following is one of his several videos on youtube.
2. Normally, such a “black magic person” can be attracted to a particular person, a place that he/she liked, or a combination of such factors. They cannot exert influence on people with strong willpower. It is possible that either she is the only “target” or the one with the least willpower among you.
– Does she practice Buddha Dhamma? If one is fully engaged in following Buddha Dhamma, it is less likely that such “black magic persons” can influence that person. Just reciting suttas is not enough. But combining those two will have a stronger effect.
– If you or an exorcist can talk to the “black magic person,” you can ask what it wants. Sometimes their wish can be fulfilled by making an offering to Bhikkhus, as I have seen in some videos by Abhaya Thero. In those videos (as in the above video), Abhaya Thero talked to the “black magic person” through the patient (i.e., your mother in this case.) When the patient is “possessed” by the “black magic person,” the words are that of the “black magic person.”
– By the way, to “take possession” of a patient’s body, either the patient must be willing, or the patient does not have enough willpower to resist.
3. Moving to a new house may or may not help. Since the “black magic person” came to the relative’s wedding, she seems to be followed, and the attraction is mostly to her, not the house. But I am not sure. It may help.