You may have to hand raise/bottle feed the runt, if she is not getting enough to eat. Go to a pet supply store and get some KMR and a nurser or syringe. Do some searching on the internet about how to bottle raise a kitten, there are some good sites.
When the kittens are weaned, please get the mother spayed. Keep her inside, and all intact males away from her, until you do, as she can go back into heat at any time.
Please spay and neuter any other intact cats or dogs and rabbits you have.
Welcome to petlovers, and do let us know how the kittens and mother are doing.