Many say Red is strong. I think he is strong, but he WAS strong in the hero party.
I remember in the beginning, he did say something like the party's strength catch up and surpass him. Maybe the hero party has become too OP and leave Red behind so far that he gradually became less useful, so that mage took his chance to kick him out. And that half asleep sister is like, yeah whatever. Or the mage is actually a villain tries to manipulate the hero, and kicks the brain of the party when chance comes.