Yes, of course! Another nursing cat's milk is no less nutritious to a kitten if it is not the mother. Letting young orphaned kittens suckle from another nursing mother cat increases their chances of survival.
Cats nurse kittens until the kittens are about four or five weeks old. At this age, kittens begin to wean and are able to eat soft kitten food. The mother cat's teets will dry and retract slightly. The only true way to tell if a cat is still nursing kittens is through observation.
No, the kittens need all the milk they can get. Just feed her better food and help her not be so thin. You can help by nursing the kittens yourself as well at intervals if they are still hungry, but the kittens should NOT be separated from their mother, especially when you can help them.
Kittens no and mother of the kittens no. The Cat should way more than usual because she is still nursing her kittens so the kittens can gain weight and grow :) Hope your new additions arent too much of a handfull