Until it's revealed we're guessing and reading what we want to into the gaps. If someone refers to an asian lady as their mum, people would assume they're telling the truth, taking it someone's words at face value. That's why Zara mentions that she expected asians - because Noji probably refered to them as hers when organising dinner. After years people get tired of long explanations, stop explaining the full background, they just go with the shortened form.
Same with Nami being a redhead is just conversation - it's not because they look genetically similar at all - Jeff, Trey and Mila thought they were a couple, not siblings. Zara assumes that because Noji is introducting her kids (even though they're wards/foster/adopted whatever) that they're related. A stranger assumes they're related is proof of nothing, just like Jeff, Trey and Mila's assumption was wrong.
But yeah, based on looks I would assume two completely separate bloodlines, just like Trey, Mila and Jeff did, and be weirded out they they have such a sense of taboo over some things while being completely natural with others. I get that they are raised together in the vanilla version, and that some people would just have a wall in their mind because culturally they've been raised as siblings regardless of biology. But as far as looks? So dissimilar. Maja and Vic have at least some similiarities, Ocean's done two sets of twins. He's just made them they way he wants and we'll find out what percentage alien Nami really is at some point.
