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They're not blood related at all. Luffy is the son of Monkey D. Dragon, while Ace is son of the Pirate King Gol D. Roger. Thought they were both raised together, therefore Luffy sees him as his brother.
^ exactly what Yankillaz said.
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One piece, are Luffy and Ace related ?
i don't know the story if one of them knows if they are related i just know about them because people seem to think they are so are they ? im not sure on it?