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You need to stop the ants accessing the room / house. you should be able to track them to the entry point and put down ant powder or block up holes. Move the whole cage to a safe room. The ants will even come to get hamster food and can be attracted to the urine even so stopping them coming in is the key to stopping them getting to the hamsters. They will have a nest somewhere and will be collecting food to take back there. You could put out a bait trap and see if they go there and then follow them going back to the nest.
A dead pup needs removed. Use a spoon and rub it in the bedding then scoop up the pup. Mum could get ill if she disposes of a pup that has been dead for a while.
If there are other signs of illness you need to see a vet.