PLEASE help me i just bought a puppy?

I just purchased a 4 1/2 month old female german shepherd today little did we know we have another 41/2 month old female and we found out they were sisters. when we went to pick up the female new gsd she was very agressive towards our other pup they only been apart for 2 mnths anyway then we walked them around they got along great now we are back in my yard and our female is being mean luckly i used to baby sit dogs and we have our yard seperated. also she is very shy and she seems like she would be mean to strangers Help thanks what should i do thanks