OH sorry I missed that part. I thought the kitten was coming from the shelter.
By the time these kittens are 8 weeks old, Tucker will be 5 months. If Tucker were an an adult cat, the kitten might be fine, but since he is only a kitten himself, he could very well hurt a tiny and fragile kitten with overly rough play.
I would get him a companion at least 12 weeks old.
I hope your fiancee's mom gets her cat spayed!