What happens when one of the unborn puppies dies inside the mother dog, will the mother dog and other puppies get poisoned?

Unless you know for a fact via ultrasound the pup is dead you wont know untill she gives birth yes the pups can get sick and so can mum but she will show pritty everdent sines that there is somthing wrong with a pup as she will have brown/green/black discharge that will come from here colver and she will through up and be very lethargic

Yep, that's why as soon as you know one is stillborn you have to take the mother dog to the vet for emergency surgery to save the other puppies.