She wasn't forced to come, yet she did. You don't think that was BECAUSE she was worried about her brother that she didn't want to abandon him? That doesn't mean she's not going to be angry about the situation. Other than genre tropes, is there anything that makes this plan make proper sense?
She did, yet she acts as if she was forced. They weren't ever that tight with her brother, and right now she's more of a annoyance than being of use.
Sure, the plan was great, but what was the alternative? Going back to the academy and forget about the whole situation. The territories are vast, the navy has probably millions of soldiers, so losing one won't count much for them, and especially won't worth all the resources to track down some unknown combatant god knows where for a single academy student, not even if she's a noble. Parents? The moment they sent them off to the academy, where obvious that they can easily die, was the moment they let go of them.
Other than that, there was the choice of do something on their own. And if she decided to go along with it, she should just shut the fuck up and bear with it. Thim hasn't whined that much, and he basically kidnapped.