Hello raddi,
I feel for what you are going through and I'm sorry you've been going through it.
I recommend the book, I Am Not Sick, I Don't Need Help by Xavier Amador who couches people on how to talk to someone who lacks insight that he or she has an illness. Anosognosia, the lack of awareness that one has an illness, is a symptom of schizophrenia that effects up to 50 percent of the people with this diagnosis.
Of all all the people I've recommended this book to [I'm keeping a running ticker now, you're number 16], your situation, I'm sorry to say, sounds the most extreme. Unfortunately, the criteria for admittance to a psychiatric hospital is that the person is a danger to herself or someone else.
Too often, it's a waiting game, and things get worse before they get better. I know it's hard for you to believe that it could get much worse, and I'm sorry to report that it will have to get worse before most staff at psych hospitals would admit her.
There is, though, one thing you could possibly do: claim your mother is a danger to your brother. I would, if I were you, be alert to any actions she takes that could jeopardize his health, and even if she doesn't get that far, I would make the case she is a danger to herself and to him, to try to get her the help she needs based on what you've told us.
I'm not exactly sure why you feel your stepdad wouldn't want to get her the help she needs, especially when her paranoia has taken over her life and that's not a healthy way to live. You may have to risk his displeasure or anger right now.
I hope you find some peace of mind and a solution to this for your mother's sake and for the sake of you and everyone involved.
Respectfully,
Christina