I believe the answer is: overattribution
overattribution refers to a social occurence when people believe they could freely express everything that they want and assumes it's true and other people would believe it. This occurence considered to be a form of bias, and make people constantly give false characteristic toward others without having enough evidence.
Rosa judges everything Serene does based on the fact that Serena is an only child. Rosa is engaging in which overattribution error.
