I know this will sound mean, but it works-
I used to have a cat named Summer that was an only kitty. She was not used to any else being around her, and she was scared to be parted from my sister's father in-law.
She didn't like to be picked up, petted- nothing. I took her home with me, put her in the house in an unused bedroom and made sure she had a blanket to sleep on, food and water, and her own litter box.
And this is the seemingly mean part- I would ignore her, go about my regular activities, and let her accept me on her own terms. I would make sure she was in her own room at night, and no other animals or people disturbed her. During the day, I would open the bed room door, and just ignore her- I let her explore the house when and how she wanted to. When she felt threatened, she would run back to the room and hide, and I wouldn't let anyone or any other animals in there- that was her room, her sanctuary.
After a couple of weeks, she would come up to me and rub my legs. I would see if she would let me pet her, and when she did, I would only pet her as long as she let me. When she ran off, I would let her do her own thing.
By the end of a month, she was sleeping in my room at the foot of my bed, on the floor, but at least she was accepting me better.
After two months, she was on my tummy, doing her milk treading thing and purring loudly. She was a very loving cat and I loved her very much.
It sounds like this cat was exceptionally devoted to your mother in law, and after enough adjustment to you, you might be surprised to find out the cat is not evil, but a very loving and devoted pet.
=^.^=