In One Piece universe, it's seem to be common to call someone you grew up with family and brother, so it may simply be that Ace and Luffy grew up together, maybe they are cousins or so.
Another theory.
Younger presented to the left and older to the right.
Monkey D. Garp and Jaguar D. Saul are related in some sense. Oldest Generation
Monkey D. Dragon and Gol D. Roger are related in some sense. Middle
Monkey D. Luffy and Portgas D. Ace, which we know are related. Youngest
As can you see there is a pattern, that the youngest "brother" has the name Monkey and the oldest something else. Now I know that this is very farfetched
, just wanted it out for opinions.