If the momcat isn't caring for her kittens, licking them, cleaning them and letting them nurse, then you need to get the kittens into the hands of a proper caregiver who can bottle feed them and stimulate them and hand raise them. If she is doing all the above and also biting and scratching them she could be ill and need to see the vet. How old is she? Is this her first litter? You can also take her in to be spayed to see if that calms her down, BUT in her absence you would have to take over the care of the kittens completely. Another alternative is to find another momcat who might adopt these kittens as her own.