1) While brain damaged, if your husband is still legally considered mentally competent--that is, he has not been declared incompetent to manage his own affairs, etc.--then he has the right to have his mother manage his affairs for him, the same way a competent adult could have a spouse, or a sibling, or a friend, etc. do this for him. Unless she has a power of attorney from him granting her authority, she can't sign documents on his behalf or otherwise act without him, but he can certainly defer to her and let her make all decisions.2) If he is mentally incompetent and had created a power of attorney before becoming incompetent, then whomever was given the power by that POA (was made his "agent" or "attorney-in-fact") can make these decisions for him.3) If he has been determined to be mentally incompetent and does not have a POA, then you should file a legal action in chancery court (a division or part of county court) have yourself, as spouse, appointed legal guardian, so that you will have the authority to make these decision, manage his affairs, etc.