I don't get why people keep complaining about this. In his shoes you would not be seeing things as presented to the player. You would be thinking "go with my siblings who I have grown up with as family, even if maybe it's true they're not blood-related, or go with a bunch of aggressive strangers who are attacking everyone I know and love?"
It's easy to sit there and judge the choice as a player where the game is clearly making Garon looks evil and you're not under any stress, but you have a perspective that Corrin does not and frankly choosing anything but Norh makes no sense.
Edit:
Now once that choice is made, the writing seems to stumble a bit with how to make an entire campaign out of that choice, because it doesn't take Corrin long to see how corrupt things are, so things get a little contrived to keep Corrin from just being able to say screw it and try to convince his siblings to defect or pull off regicide. However, that's really a different problem.