I am sorry you are having a stressful time with this. In my experience, when I brought Paizly home Taz was not happy. But I let them work it out. BUT Taz was not aggressive and I didn't have fear that he would hurt Paizly. Taz never did "like" Paizly. He still growled when she came near him. She never growled or hissed at him though. She always tried to play with him, attacking his tail when he was sitting on a chair.

Now that I have G, my friends cat, the tables have turned. Paizly doesn't like G and G isn't crazy about her either. When I let G out of his room he will chase Paizly until she goes under the chair or bed where he can't get to her. Then he will sit there waiting for her to come out. That's when he goes back to his room. So sometimes he is only out of his room for 5 minutes. I've had G since August 10th. His mommy should be coming to get him soon. So I can deal with the situation temporarily.

I can't tell you what you should do. You do what is right for you, Schroeder and Aroara. I will keep you in my thoughts that you can figure out a solution that is best for all involved. I also hope you find your sisters cat soon!