Sad about my 3 Month Old Yorkie

Question
I just brought home 3 month old Yorkie sisters. One is smaller and was shy, the other was outgoing. Yesterday I bathed them both and from that moment on the smaller of the two keeps attacking her sister. This is not playful biting, she is being mean. Now the one that was outgoing is afraid of her sister and is acting shy. I separated them and tried to re-introduce them to one another this morning but the little one still has her nasty attitude. What do I do?

Answer
Unfortunately, it isn't usually a good idea to purchase litter mates, especially same gender litter mates, and most reputable breeders don't sell pups this way.  
Even if you can get them to get along now, there's no guarantee that they will once they reach social maturity, and if someone asked me which I'd rather have fighting, males or females, it wouldn't be females.  Males tend to fight ritualistically, but females can get downright serious.  
One possibility is that you accidentally convinced one of the pups that her sister was responsible for her getting dunked - that would be if you bathed them together.  Or, it could be the fact that their scent has changed.  Or, it could simply be that they didn't like each other, or were competitive in the litter, and were just too new in your household to act on it at first.
One of the best ways to get dogs to get along in a group is to have the human occupy a position of strong (but benevolent) leadership.  If it were me, I'd take these dogs to puppy class now, and in separate classes.  Then, I would institute a program of NILIF (Nothing in life is free - Google the phrase) at home, making each dog "ask" permission for everything they get.  (Wanna go out?  Sure.  "Sit" first, then I'll open the door.)
If all fails, including getting a good positive trainer or behaviorist to help you, then you may want to consider getting one dog a new home.  It's not fair for any dog to live in fear its whole life, so if that turns out to be the case, opt for having the dog go somewhere where it will be loved and cared for, not snipped at constantly.  Good luck, and let me know how you make out.