The kittens and 'mum' need to be taken to a vet NOW. They will be able to determine if the dead cat is mum, keep the kittens warm and fed, or if cat is not mum, advise about what to do.
Food is important and equally so is warmth.
You shouldn't really delay with faffing round with rescues, that can be done once the kittens are safe - especially if you are thinking of maybe raising them.
Hand rearing is very hard work, not very successful and I would advise you to have someone to help so that you can get some rest in between, unless you are lucky enough to find a nursing mum.