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I am sorry to hear about this situation. Without a medical power of attorney or living will, the default succession law falls to immediate family. First, the spouse, then the children or parents.
However if the spouse is the one that caused the medical state, then she is disqualified as the immediate relative who makes medical decisions. As such, she should have no right to make any medical decisions for your brother due to her actions. If necessary, a Court can formally void her ability to make such a decision if his sons or you file to formally invalidate her as the "next" in line to make medical decisions.
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