puppies/ aggression

Question
I have 2 16 week old puppies. They are sisters and are a cocker spaniel (mom) and (probably) border collie mix. One is black and looks just like the mother and the other is tri-color.  The tri-color is very aggressive to the black and at first I thought she was asserting herself as the dominant one. The problem is that the other one is not backing down and the fights are becoming serious and I am very alarmed. This started about 2 weeks ago, but I can recall nothing that might have triggered it. It appears that the Tri-color is jealous, but we have given her no reason to be.
My daughter and I love these babies very much and would appreciate any suggestions or ideas you have.
Thanks
Karen

Answer
At that age I would doubt it is true fighting.  Is the hair on their backs up, noses wrinkled, and quick short snaps?  If not they are just playing.  The recent start does worry me.  Play begins almost as soon as they are mobile.  

Talk to the vet about spaying them.  The trend is toward doing it earlier and earlier.  It could go a long ways toward solving the problem.