Ok so here's my theory that's probably been said a million times already: Ruby is the Spring Maiden. Summer was the former Spring Maiden, which is why Ruby doesn't know, she's been a maiden all her life so that power either is dormant or used subconsciously - and she was let into Signal & Beacon due to both Qrow & Ozpin knowing she'd need the skills to protect herself eventually. Expanding on subconscious use of the power - that's how she kicked that griffin into the ship after it ate Roman, and why she took no damage during that fight. No normal person could have managed that without even a fricken scratch, and then as per the preview she's STILL FIGHTING and still unharmed by the point everyone else is tired and hurt. That's not normal endurance and durability.
As for Maidens being linked to Kingdoms - I don't think they are. Ironwood was the leader of Atlas prior to being pulled into the 'circle' with Qrow & Ozpin & Glynda, so it goes to think that the Maidens operate independently of the cities, and are not known by said cities. On top of this, Amber didn't seem to be in Vale prior to her injuries, just near Vale (maybe not even that, since she had to have gotten to Atlas pretty quickly to not die prior to the machines being hooked up to her).
Also, remember that the Maidens are able to choose who gains their power when they die, if they have the mental frame to do so or if they die peacefully (old age) so it is entirely possible that a family could pass it down over generations in order to keep it safe and under their control. I doubt it's the Schnee family, since they have summoning, but... well, perhaps the summoning ability was originally part of Winter's power (since it seems to be ice-related) but broke off or slowly melded into their own semblance and auras, becoming part of their blood? Could mean the same for the Branwen family, hence Yang, Qrow, and Raven all having the red-eyes-take-warning feature, but with the females being much more red-eyed than Qrow has shown thus far.
Beats me though, I could be entirely wrong xD