Her mother told me that account has nothing to do with my brother. My sister-in-law says it's for HER retirement. What rights does my brother have to that income if they are married? They are both mentally challenged but my brother if able to maintain a full-time job. Is he being taken advantage of if he's paying all the expenses except half the mortgage and half property taxes?
From a practical perspective, does your brother know that his wife is perhaps taking advantage of the situation? Maybe he looks at this as forced savings that will benefit him and his wife one day. He does not have rights to the income if it is payable to his wife.
My answer is not intended to be giving legal advice and this topic can be a complex area where the advice of a licensed attorney in your State should be obtained.