Introducing puppies to older dog

Question
We have just brought home the 2nd of the new puppies we have adopted. They are sisters we brought one home last Sunday and the other today.  The one we brought home on Sunday was the aggressor and tried to hurt the newer one.  We also have a 2yr old German Shepherd who accepted the first puppy without any problem.  But if the one puppy is an aggressor will the german shepherd also be aggressive toward the new puppy.  Also, will they now attack the cats.

Answer
True aggression in young puppies is rare.  More likely what you are seeing is just the common rough play most puppies enjoy.  I am surprised the first puppy and the 2 year old Shepherd haven't been very actively playing all week.  Usually puppies will see cats as more potential playmates and try to engage them in play.  Usually cats want nothing to do with such.