Puppy Sisters Fighting

Question
We have 2 nine week old english springer spaniel litter sisters. one is the runt and the other is much bigger. They quite often chase each other and try to bite/nip each other. Is this just play fighting, are they fighting for dominance or are they becoming aggressive? should we always seperate them or leave them to sort it out them selves?

Answer
Most of the time this is perfectly normal esp in pups, and is nothing to worry about. I would maybe spay the larger one to be safe in future. Just keep an eye on it, get them plenty toys and encourage them to play with the toys instead. All pups nip, and get rough with each other, just watch out for real injuries.