Why is my cat so mad?

Okay so we bought a new kitten 2 months ago and now she is bright and playful and really sweet. So my sister decided to get a new kitten who is 5 weeks old and brought her into the house. They live in the same room and evrything but do not share litter boxes or food bowls. ver since the new kitten cam the old one hisses at it and us! W cant evern pet it or pick her up because she hisses and growls at us. I have heard they are territoarial but why is she mad at us? And she hides under the bed and dresser. How do we make this change easier for the both of the?