User blog comment:Dany1459/Monkey D. Dragon and Gol D. Roger are brothers/@comment-25270312-20161130011438

A theme in One Piece is that adopted family is just as much and sometimes more than blood-related family, so I find it unlikely. Garp did frequently drop in to see how Ace was doing and has known him since he was a baby, so he's every bit as much Ace's grandfather as he is Luffy's. I don' t think Garp considers promises to be worth less than blood.