My cat has lost a lot of weight. She won't eat. The kittens are seven weeks old. They are fine. I'm just worried about the Mother.

You should get her to the vet ASAP. Cats can develop hepatic lipidosis when they don't eat.
Although, there is possibly something else going on that has led to the lack of appetite.
You need to get her to the vet, medical help is her best chance.