Have the same issue. After completing the brother rescue quest chain, I went to the nearest town he was staying at. Through town menu, selected him and chose, add hero to party. He appears in the character sheet screen (c) and party organization screen (p) but can't be accessed in the inventory screen, as the other companions can be. He simple doesn't appear there. When trying to form a party through the clan screen, it reads that he's busy with a quest. Uploaded the save file to the link above.