The crossword solution for the term 'Treaty Brother' is 'ALLY', which is a 4-letter word. An ALLY is someone who is connected to another person or group by a mutual agreement or support. In this case, an ALLY is someone who is connected to another person or group through a treaty. A treaty is a formal agreement between two or more countries, organizations, or people. Therefore, a Treaty Brother is someone who is connected to another person or group through a treaty.