It's mostly justified compared to other series with the same overused trope. MC here dragged no-level Sister who has zero combat training as a pure civilian and turned her into a literal cannon ball. Not only was she already mentally strained, being sent there as a sacrificial messenger (the church folks don't care if she gets killed as a messenger, hence they chose one that isn't combat capable, and most likely would've wanted the messenger they sent to get killed anyways as it can be used to start war as the "good guys"), she is also heavily confused by what's going on with MC and co.'s antics. And then she gets fired around as a projectile with no power to refuse, towards monsters that are obviously a huge threat to people like her. Of course she'd be mentally broken.

I know, it's just that the way they treat the MC disgusts me sometimes, everyone heard the Sister's situation and imagined what would come next, they went out to do what they thought should be done (get stronger), but no one other than the MC really cared about the Sister and tried to find a way to help her, what he did was abrupt and perhaps exaggerated but it doesn't change the fact that he made her take the first step to make her change