It translates to "My brother was once the crown of our father (Akatosh)". It appears that "du'ul" is not being picked up by the Translator. That's an error. If you look up the word in the dictionary, it gives "crown, symbol of authority, rightness". Du'ul also is referred to by the Graybeards in the blessing they give you after you return the Horn of Jorgen Windcaller to them - "strundu'ul" - Storm-crown.