Ginty's being dismissed in this thread for not spending time with her family, family who mostly don't seem to want to spend time with her, unlike Patrick who does, as well as avoiding household chores. Peter's resolution to not dance with his sisters means that Ann ends up as a wallflower, when she might well have enjoyed a dance. It might be typical of a teenage boy, but it's also entirely self-centred. So no, it is not unfair to Peter to condemn him for avoiding his family in pursuit of his own pleasure/needs, as well as his laziness in not taking on his share of the allotted household tasks.