[@BurningDaisies]
[@Genkai]

If we're not playing the royals themselves, there's an interesting case for having our chars be the retainers of the Nohrian royals, what with the whole idea of turning against your friend and liege and all the fallout from it. Bonus points if Genkai is playing the Hoshido siblings as royalty because that's perfect fuel for misunderstandings.

But anyway, for your questions (answered assuming royalty as per above):

The character I've got works best as the heir apparent, so he ought to be the oldest yeah. I didn't really have anything in mind insofar as extra siblings, though.

With that in mind, if they know, the family reaction is probably going to be pretty adverse? You've got the best and the eldest running around god knows where, hanging out with national enemies. 

But if they're supportive of our characters, that would open up some interesting avenues. If they don't believe us but still support us, maybe they're secretly Hoshidan sympathizers/spies or something. If they do believe us and support us, then that's that. But I imagine putting resources into a project that is traitorous if not insane would not fly with the severe nature of Nohrian politics. 

If they don't know period, I'd imagine they'd be worried sick and probably send some people after them or assume the worst and take the appropriate courses of action (whatever those might be)