The term Akainu uses is "Kyōdai" ("兄弟" in kanji, read as "きょうだい"), which can be used for both blood-related siblings as well as in the sense of "friend/colleague."
I believe that in this case, Oda uses the term in the same way he does for Luffy, Ace, and Sabo. For example, Chapter 585 is titled "Brothers' Sakazuki," which in Japanese is "Kyōdai Sakazuki," and it's the chapter where those three become brothers.
Source: Redon