The prince is even more of a jerk than you think, you'd think he would talk to girls and seduce them away from public sight, but according to flashbacks, the prince constantly ignored Melissa even during public events, particularly during balls or dances, where he would spend the entire night seducing and dancing with other girls and ignoring his actual fiance, you even see a shot of Melissa crying in the middle of the room as everyone dances around her, including the prince and one of his girls.
This makes me think the guy is incredibly charismatic if he can do all of that in the open, dance as his future wife is crying and convinced everyone that all the fault in his constant cheating lies entirely on Melissa for being mean.
And yes, the father and brother know all about this and also blame her, it's a world of jerks to Melissa.