Dragon and Rocks look very similar so I have a theory that they might actually be father and son. Dragon is 55 right now and the God-Valley War took place 38 years ago meaning Dragon was 17 back then which means he may not actually Garp's son but his son-in-law. So technically, Luffy could be Rocks' grandson. It may not be vital information but they might reveal it in like the last arc or guidebooks as "fun facts"
Also Ace was kinda adopted by Whitebeard at 20 so Dragon at 17 being adopted is plausible
The hair type is different but the hairstyle is similar.
Their face shapes are also similar if not the exact same
The jaw and their eyebrow ridge do look similar too
They could also be brother's or Garp is really Dragon's dads. They may not look like each other but it's One Piece, Big Mom and her children don't look the same as well as Kaido and Yamato and Dragon and Luffy