How to train an avoidant cat to be friendly

My mom is giving my sister and I her cat, cookie, because she has two (including him) and she doesn’t like him :(. From what I know , he has always been to himself and she has never stimulated him by getting him toys since he was a kitten. For context, his mother was a stray and we took him in from a friend of a friend.

Whenever I visit, cookie takes a while to open up but he would slowly brush against you and sit next to you. However, he doesn’t let you pet him often.

My sister has a cat so I’m afraid if cookie doesn’t get along with the cat. Cookie has a brother that he play fights with but they both are extremely under stimulated at my mother’s place. She only feeds them… honestly, I think they both look unhappy.

I really want us to take cookie to hopefully give him a happier life and play with him so he can be an active happy 2 year old cat.

Please see vid attached of my mom petting him and him attempting to bite her…I’m afraid he’ll do that to my sister and I lol