Helen was naturally older than him, having been born several years before he was ushered into the world one dark and stormy night. Always the youngest, never the oldest. (The children hardly counted.) There was never supposed to be an age gap like that between them. He'd assume she was simply from a time well beyond his own, as their respective lifespans made such a thing possible, if not for something she said earlier.
Nikola's more than clever enough to put two and two together.