Older cat acting strange since we brought kitten home.

[sassy_n_steeler]

Hi. I'm Anna and new to this site. I have two wonderful cats, a 1 y/o Calico named Sassafras (Sassy) and a baby silver mackerel tabby named Steeler. Prior to bringing Steeler home three days ago (we rescued him), Sassy was our only cat and was very loving and gentle. However, after noticing a new kitten in our home, Sassy does nothing but hiss, growl, and attack my boyfriend and I. After going for Steeler's face, we decided to isolate him from her as we are very afraid that she could potentially kill him. After three days, her behavior has taken a change for the worse. She has started to urinate all over our house (something she never did before) and she will not let either of us pick her up. We are thinking about finally integrating Steeler with her, as we want them to become "friends" and want her bad behavior to stop. Is integrating a kitten with a cat a good idea, or will she kill him? Or should I keep him isolated for a while until he grows a little bit stronger to defend himself? I am completely stuck here and it has created a great deal of anxiety on me. Thanks very much!