Help! What could have killed my two fancy goldfish (please read) ?

Sorry to hear about your buddies, I know I shed a tear for lost pets. Unfortunately you may never know what happened. Your brother may know, he was the one in charge of them, until he comes clean and tells the truth ( he may feel really bad as it is about them, so he may not want to make you mad or ashamed at him) you won't know. I would sit down with him and have a calm talk and ask again about what happened, tell him that your not mad, just disappointed and sad about what happened and would like to know the truth. Hopefully you find out what happened. Oh....I would find a new fish-sitter.