It shouldn't be forgotten that Aya Zane—and no one hid or concealed her, nor did she change her surname—when people with such authority could have easily done so. Why then would they have changed Niki and Nami's surnames if they were Zanes, bastards who were completely abandoned? No, that's too far-fetched.
I'm inclined to believe that Niki and Nami are war orphans—children of the defeated, perhaps Asaver's, etc.—or children of family friends. Monica and Ben could be Nami's parents, while Niki... well, he's a stray dog, so to speak, since he can't be a child of the Rivera or William households for the same reason as Aya Zane might be Zane. There's no point in changing a surname and hiding for 20 years.
But you are right, there is too little information yet.
PS
Or they could be the children of Dialan and (Helen?) when Dylan in a dialogue with William says "we lost two...". + Dylan has red hair, green eyes, Helen has black hair in places, gray, almost blue eyes.