* If the mother cat was a stray: Often, the mother cat is trapped and spayed/neutered after her kittens are adopted. She might be returned to her colony, placed in a foster home, or adopted herself.
* If the mother cat was a pet: She might stay with the family who adopted the kittens. Or, if she was a multi-cat household, she might be placed in a new home with other cats.
* If the kittens were adopted from a shelter: The shelter might try to find a suitable home for the mother cat as well.
The most important thing is that the mother cat is cared for and finds a loving home, whether it's with her kittens' adoptive family or somewhere else.