About this whole Ace is Luffy's brother, Luffy's father's Dragon, Q. E. D. Ace's father's Dragpn-argument…
It makes a good argument, but still, try to remember One Piece-terminology. Nojiko is Nami's sister and Bellemere's her mother. Non of those three are even remotely related to each other, yet considered each other their sister/mother/daughter.
It could easily be so that as of them both being D.:s and growing up together and all, Luffy and Ace consider themselves brothers.
Furthermore about "Thaat's impossible, Shanks isn't a D!", a reminder, right up until chapter 433, neither Dragon nor Garp were D.'s, and, right up until Chapter 154 Gold Roger wasn't a D.
Hey, we honestly have no proof at all for rightiously claiming that the Gurosei aren't D.'s.
Everything we can conclude so far, is that Shanks and Ace for some reason resemble each other in appaerence.