I don't even think there's any implication that they both lost their parents in the same accident. The only thing that's mentioned explicit in-game, as far as I can recall, is that they were both 3 when their respective parents died. This is what the MC told Maja about their parents in Chapter 3, which as far as I know isn't being reworked:
"I lost my parents at the age of three. The same happened to Nami."
If they were siblings, it would make more sense to say:
"Nami and I lost our parents at the age of three."
If it were the same incident, it would make more sense to say:
"I lost my parents at the age of three. The same incident killed Nami's parents too."
The way it's actually worded, if there's any implication at all, it would be that MC & Nami aren't related, and they lost their respective parents in separate incidents.
But then again I have no real insight into what Ocean's true intention really is. So take my interpretation with a grain of salt.