Is the mother cat friendly to people? If yes, she might have bee

If you make a warm little shelter for her, she will take to it quickly. If she's afraid of people, then also make a warm little shelter in a hidden place, place some cat nip in it.  She'll be attracted to the shelter and will probably stay to raise her kittens.  When the kittens are old enough to be weaned, try to get them inside your house, socialize them, and find good homes for them.  Also, try to fix the mother cat when she has finished nursing the kittens.