Brother: أخ (ax).
My Brother: أَخي (axí).
Just a side note, the word أخ is one of the special nouns in Arabic. Sometimes when adding possessive suffix to its end, it is required to add one extra long vowel. Example: His brother أخوه (axúh) - The letter Waw و is added here between the word أخ and the possessive suffix for "his" ه. This vowel would change as well when the word comes in different statuses like Genitive or Accusative and so on.