If you're your brother's legal conservator, then you can take legal action on his behalf. You may consider it in his best interests to seek a restraining order against his ex-partner.
But if you don't have a court order authorizing you to be his conservator, you can't. if he's competent to make his own legal decisions and he chooses to see her, then you have to respect that choice, even if it makes him ill. If he's not competent, the first thing you have to do is get yourself appointed his conservator.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.