You may just need more time. You need to spend time with Isis to let her know that she's still important. Isis is behaving the way small children behave when a new baby comes home and they're no longer the focus of attention. However, if poor Camile is going to keep getting the crap beat out of her by Isis, that is a miserable life indeed for Camile. And rehoming would be the kindest thing for Camile. I've never had much of a problem introducing new cats into my home. However, when I brought Bogart, a stray into my home with Garfield and Augie. Augie didn't much care but Garfield took great offense. There were fights that would look like something out of a cartoon with the two of them snarling, growling and tumbling around my apartment. I remember having to grab a broom to chase them both out from under my bed a lot the first few months. But eventually, the two of them became very close and would take naps together.