I can't stand that MC. Professional speaking, that asshead magician had a point, but he's a bastard and you also have to draw other points than just strength into the equation. That bastard is no team player, but a back stabbing bitch. This makes the mage himself is a danger for the party. An enemy within.
So, MC can't keep up any more and should leave the party. Okay. Let's discuss this with the inner core of the hero party (not the whole army) and give the reasons. The only reasons I can think of, why the MC didn't do that, is that he tried to avoid to harm the hero's party inner structure and he might have wanted to spare his sister's feelings. But that mage guy is the reason for this conflict, not the development that they surly have been seen before they reached this point. That mage endangers the peace of the party. If MC has to leave, so should the mage also. If the MC is no ally because of his lack of skills, than the mage is also no ally, because of his lack of common sense. But by not pointing out, what kind of game the mage is playing, the MC endangers the whole party.
Also he didn't gave his own sister a warning about that bastard.
Also the came to far, because the MC was there, so every part of his equipment is earned. Both MC and mage knew that, but MC let himself be blackmailed by that guy, so that the mage keeps his trap shut.
So, the MC wussed out and made a shitload of terrible mistakes. I hope the author gives a damn good explanation, why the he acted the way he did, because I can't see logic in it. The only explanation so far is, that he holds the idiot ball.