Fighting!!

One thing you might want to take into consideration....separating them, rehoming, etc. is not "giving up on them".

Not even close.

It is doing what is best for the dogs.

So, if you do have to eventually rehome one, know that they both will be happier, and that you did what needed to be done to care for their emotional well being.

Actually, if trainers had no knowledge of this type of problem, we wouldn't groan as much about the breeders (and I use the term loosely) who would even sell two bitch littermates to an unsuspecting person who just wanted a couple of nice family dogs.  Whoever this breeder is hasn't enough dog savvy to be breeding anything...[:@]
The advice our OP has gotten from mrv and others who advise separation and NILIF, etc. is good, but if, as Gina says, the two bitches develop a dislike for one another, unlike males fighting, which is usually ritualistic, females can get very serious and actually kill one another.  Not a laughing matter.  Consult a behaviorist or trainer who is skilled in aggression issues, and steel yourself for what may be inevitable - you really might need to rehome one of the dogs.  Next time, don't get littermates at all, and don't get dogs so close in age - you are stacking the deck against yourself if you do.  Some dogs can live amiably in a pack, but if you are a newbie, stick to those parameters.  It's safer, and less heartbreaking.

Good point about the "breeder", perhaps it was an accidental litter?  Maybe whoever it was was simply ignorant rather than uncaring.... If it were me I'd get back to them and let them know the problems I was having and that they shouldn't have sold sisters together.....  You never know, it might get through...... what am I saying......