Raiden doesn't question it. Technically he didn't even have parents at all, and yet he had a brother. He glances at that tattoo when Minoru gestures, and that helps explain why he had as much ink as he did. Not that Raiden didn't know mortals who had more than that, and only for ornamental purposes.

"I used it often to travel between the realms. Unfortunately, here I can only move short distances with any predictability."