That is a beautiful cat, she's part of the Siamese family and I see that Cassie is as well.
What you need to do is make Cassie know she's number one, it's all about her having her space. Does Cassie like to be in high places in the house? If she does, then you're just fine. High spaces, meaning does she have a cat tree to sit on and watch things from above.
I have the problem too, I have a Seal Point Siamese and she's the Alpha Cat. I'm trying to help her get along with the other three, a boy and two other girls. No matter what I do, Suzie just wants her space. As long as Cassie and this cat don't touch each other; ie when it looks like they're swiping at each other and they have a good distance between them, then you should still be fine.
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