Portgas
D. Rouge and Gold
D. Roger is incest?
That explains why Ace was so retarded . According to cousin theory
Nah. Even if the D's are related, their common ancestors are way back 900 years in the void century. Thats too long to matter in genetics shenanigans. While they could have other common ancestry, there's no proof or indication of that so far.
And even then, there are giants with the D initial too. So unless a normal human fucked a giant(totally possible) , the theory that D's were originally a pirate crew is more probable.