here's a thought that might help... maybe her sense of agency (job, purpose, reason for being created, how much control she has and all that kinds of stuff) is to be the denial factor. and maybe being psychotic is just part of who and what she is (sense of agency stuff)

my suggestion is just continue being you and let her continue to be who and what she is... kind of like a friend of yours that you visit and go see and do things with you would not ask them to change who they were right, you wouldnt expect your friends who do not reside with you to be the same as you inside and out. well this psychotic alter is not going to be the same as you, they are going to be different just like people are different and not the same.

what it boils down to is everyone can only control their selves, and be their selves so the same with alters, maybe accepting that this is who and what she is and you are different and you cant change who and what she is, just go on with your life how and who you are and how and who you always have done. this might take some of the stress of you. so what she thinks you all are the alters and she is the biological human, doesnt change anything you all are who you have always been. someday when the time is right she will figure things out on her own if thats part of who and what she is.