Looking for a fantasy book I read years ago

I read this book like 10 years ago and I have no idea what it's called or who wrote it. I do remember some of the plot though!

The book follows the royal family (i think) of a kingdom. The father disappears or goes off to war or something. The eldest son goes to look for him because he keeps dreaming of this weird corridor and ends up at this really strict monestary learning a martial art. Meanwhile the mum, left taking care of the infant you ger brother, is made to get into a relationship with this other sleazeball who gets like assigned to be her husband or something? She's not thrilled about it. Eventually the older brother rebels against the monestary and defeats the other students with a martial art he invented. He finds the corridor he's been dreaming about and the book ends in a very literal cliffhanger.

I remember there being more books in the series but I was so angry with this cliffhanger that I never got around to them. I'd like to gove it another go though! Thanks for any help ^