@Gore:
Also. Doflas mum just died. He killed his father in 2 years. So something else must have happened and built up to it
Chronologically, I would say that the build-up (and probably the murder too) comes right after the opening scene of this chapter (Doffy's dream). The house to which Doffy, Roci and their father are tied doesn't look like the one they first land in, the mother is missing, all three are wounded…This seems to be a completely different scene compared to what we're shown in the later flashback.
As for the D, Law being one isn't much of a surprise. However, taking in Cora's warning and Doffy's earlier regret at not having had the Ope-ope on "that fateful day", I would hazard a guess that the "Celestial Dragons' treasure" has to do with the D people. Anyway, I hope Oda doesn't leave us hanging next chapter. There's never been a better time to learn more about the D.