That actually made sense you know...back when the game actually made any sense.
But when the retcons began

, all shreds of logic and common sense went out the window. Still don't undertand why he won't rework the entire S1 script, remove those ambiguously stupid lines that don't fit the story anymore and actually create a believable background for both Nami and the MC.
I know right? It's like dealing with a bipolar writer with all the mixed messaging.
Long story short, they are siblings unless a publishing platform asks. In that case they are "non-sibling childhood friend-roommates" in a world where Child Services doesn't seem to be a thing. They were brought home together by Nojiko not-Cyrus Miyazaki on "Pick an Orphan off the Street Day" after both losing their parents in the same way at around the same time. If any of that sounds weird, unfeasible or convoluted, you must not be be a brilliant writer like Ocean.