I think it went down like this,Gold Roger had a kid,but he gave it to Garp.Then Dragon had a kid and he was also given to Garp.Ergo,they were both raised like they were brothers.When Ace was in his cell and he was visited by Garp,Ace said that both him and Luffy had a criminal's blood in them.I guess it wasn't the same criminal blood.So I guess Luffy and Ace aren't related by blood after all.But family ties aren't weaved by blood,they are weaved by love.(Heard this crap from Desperate Housewives)

IMO,the fights in the manga(coupled with my imagination)are much better,since in the anime the animation can get pretty crappy.And don't get me started on the coloring!.It hurts my eyes.

BTW,don't forget that Rayleigh has the special Haki too.