3 royal siblings- eldest heir to throne, daughter assassin,bro soldier riding a giant bird

This description may be a little scattered. Set in an Asian-esque culture, three royal sibling are coming of age in a kingdom on the brink of crisis, though I cannot remember the exact nature of this crisis. The eldest brother is training to be a sort of monk and the eventual heir to the throne. I believe the sister is training to be a sort of assassin (this one I'm not as sure of as I'd like to be). The other, younger brother is training to be one of the kingdom's most elite soldiers that ride a giant bird, I think resembling a hawk. This brother, in his training, experiences some mysterious interference during his training. A noteworthy moment is when they are training to untie another soldier while underwater and the knot he is supposed to untie is much more difficult to untie than its supposed to be (the author makes this moment more exciting than I did).

The Emperor’s Blades by Brian Staveley

Thank you very much!