User blog comment:Dany1459/Monkey D. Dragon and Gol D. Roger are brothers/@comment-71.13.82.131-20161129155917

Based upon your idea Roger would either be Garp's son or as others have pointed out maybe Garp's brother. In either case it would seem implausible that none of this was ever spoken. As Garp seems to like to talk, just as he so easily told the SH crew about Dragon being Luffy's father, he would have said this as well to people close to him. Since Luffy and Ace acted as brothers anyways, he would have said as much to them years ago as it wouldn't have changed their very strong bond.