Will a 2 weeks old puppy survive away from its mum?

my sister in law has just rescued a 2 week old puppy as its mum has rejected it, she has another dog who has started mothering it and she is bottle feeding the pup. i just need some advice on this of what my sister in law should do and why the puppies mum would have rejected her other then the reason being that she was the runt of the litter....