Even me, with my complete lack of imagination, can come up with several retcon explanations why Noji, Nami, and Nika live together without being related to each other. It may seem far-fetched or implausible to some, but then again, who cares. People are perfectly fine with Spiderman or House M.D., even though both are pure fiction that can’t stand any serious criticism. As long as people like the story and the characters, they’re willing to suspend their disbelief to the maximum.
"but we all know the truth" - It's a fictional story in the making and the author is known for changing it on the fly. We know shit.