That is definately a sure sign. It always helps to have a picture or object of the intended target. From all the signs and listed info it surely seems that we know who the practioner is and that she has malevolent intentions.
Finding a solution is another problem. It sounds like you are already in conflict with your mother in law for many reasons. Although it will be stressful try and confront her with your knowledge of the problem and that you suspect her of Witchcraft. Tell her to stop and recind her castings. The problem is that she will deny any and all activity as i have encountered this situation before.
The problem with you as an absolute beginner is that I could recommend several rituals for protection and shielding even a witches bottle or ball. But because she has powers and she has obviously been practicing for some time and has an edge in the occult it might not prove effective.
However you might try some shielding rituals which will cut off most attacks. Here is a good link to create some shields and wards that gain more strength over time as you continue to work with them that might help beat off her attacks.