There's something that makes me really interested in this flashback that we're about to receive.
In a scene 2 chapters ago, Dofla narrated very very fast how he had no "biological" family left, and how his new family and his most precious thing in the world were now his crewmates.
The way the narration went was "My mother died and I killed my father, now I only have these guys left". This chapter, we learned that, according to Law, he also killed his brother. Question: why skip that? He already admited he voluntarily killed his dad. Did he feel that killing a brother is more evil than killing a father? Did he think that telling them he also killed his bro was going too far? It feels weird to go through the reasons why you're family-less at the moment, and skip your brother.
I think we can discard Oda forgetting about it or thinking about it later; Corazón, at the very very very very least, was created (or thought about) 20-25 chapters ago. Oda had him in mind FOR SURE during chapter 747. So we have to assume he voluntarily skipped that part of the story.
I think the question now is: why? I think that's worth theorizing about. We can add up the instances were Dofla seemed (or at least appeared) mildly puzzled as to why Law betrayed him. If he knew Corazón was very dear to Law, and he voluntarily killed Corazón, wasn't it easy to put two and two together?
I think there might be a chance of being some sort of confusion or something. . .but I wouldn't bet on it, personally. Dofla, so far, is a pretty straightforward evil guy. Excentric, yes. Loves his (close) family members, yes. But evil as evil can get. So I guess, if he had no problems killing his dad, he could kill his brother as nonchalantly. But, just saying, there's some ground to think that, maybe, things aren't as clear or telegraphed as we think they'll be.