I can relate @Hopeless. I am up against a similar (denial) mind-set in my in-laws.
The only thing I have found as a way forward in this sort of situation is to hold to my own integrity, and let the truth emerge by degrees. I leave off trying to bring them around to my understanding by agreeing to disagree. You begin to recognise that point where you just can't get through, as you are expressing. Although it's tough, you really don't need battles on two fronts.
If you can agree to disagree, part of that agreement will be that you continue to support your mother and brother in the role you believe to be appropriate, and the other part is that he (your husband) refrains from repeatedly expressing his point of view in the face of what you are trying to achieve.
He (your husband) will only hear about the situation in increments. Effectively he will have side-lined himself, and although it means you don't have much-needed spousal emotional support .... well it ain't happenin' at the moment anyway, and at least you won't have spousal emotional opposition going on.
As the doctors make headway with your brother, and the treatment begins to take effect, your husband will come to understand in hindsight that he was wrong, and ought to have been more supportive. Hopefully that will happen sooner rather than later.
I can't speak for mental illness across the board - my experience doesn't extend that far - I can tell you that my husband is harbouring some sort of awareness about his condition. Last year he made the comment that the psychologist I was seeing for emotional support "would love to get his hands on me .... I would be a psychologist's dream ....", talking about himself.
When I repeated this comment to the psychologist, his response was, "Psychologist's nightmare more like .... !"
The point is, even though it was issued in a joking form, my husband was displaying an awareness that he's not okay. Even in the midst of playing up, your brother also appears to be aware that there is something wrong with him. It does muddy the water a lot when there is manipulation going on, but as you said, that doesn't mean he's not ill. If anything, it's a clearer indication that he is .... well people don't behave this way.