Ha how sad, I was looking for One Piece discussions on this topic and so I joined just to put forward my little theory. So hey guys! Gomu Gomu-First Post!
Luffy and Ace, despite not being related by blood, do look alike, and lets not forget how alike Luffy is to Roger. What if they did share the same blood, in terms of cousins?
Roger could easily have had a sister and married or had relations with Dragon. As lets face it, the man who is now the most wanted in the world MUST have passed by or had dealings with the Pirate King. So if that was the case, there wouldn't be difficulty in Dragon meeting Roger's sister. I can imagine her being very stern in comparison to Roger's antics and Dragon being the more serious man he is, probably being attracted to that type of woman.
If we went with this theory I can see it going during the time that the Marines hunted down anyone who could've given birth to Roger's son. Is it that much of an impossibility that they also hunted down any relative? But imagine one relative was protected by the Revolutionaries, and the other in protection of Garp? Roger might have entrusted Ace to Garp due to coming to trust the Monkey family as a whole. Knowing his sister was under the protection of Dragon, which I believe she still is. As for not being there for Luffy, well, Revolutionary work and the fact if she was to make herself known, then there's a good chance she'd be hunted again.
It has also been stated by Word of God (Oda) that Luffy's mother is alive in a SBS question.
So maybe that is why Ace and Luffy do look alike and Luffy resembles Roger so much, because he is, in a small way, a part of that bloodline!
