I agree with Laura. When my cat had her kittens she was a very loving and good mother but a little while after they was weaned from her she became grouchy towards them treating them just like your mother cat is doing to her kitten. I still have my mother cat and her now 2 adult kids to this day, she is now 12 years old and her 2 adult kids will be 11 in July, and she still dislikes either of her adult kids near her but to be fair they arent very nice to their mother. They bully her often, which I put a stop to when I am home. So I dont blame her for hating them. She is a gentle, kind, and loving soul who just wants to be left alone by them. But that doesnt mean that your mother cat and her kitten's relationship wont change and they get along in the future. But it wont be a mother son loving relationship, it will likely be more like a roommate friendship. Give Laura's idea a try and see how it goes. Its quite likely they do not even realize they are related anymore.
It was very kind of u to take her kitten in then his mother too, I hope things work out and they get along good soon :)