When you try to introduce kitties, you need to do it gradually. By that I mean, keep the new kitty isolated in a room with her food and litterbox. Take a towel and rub her down with it and let the resident kitty smell her scent. Do the same thing with the resident kitty. After a couple of weeks, put up baby gates stacked on top of each other in the doorway so they can see and smell each other, but Isis will be protected. After THAT, let Isis out SUPERVISED, standing by with a spray bottle in case violence erupts (by violence, I mean bloodshed). Only let her out when you are home and can supervise.

It's not something that will happen overnight, so please be patient.