It would have made much more sense for them to kill their parents

Just watched the film. Their parents, especially the Mother, were their oppressors and induced their misery. Even the psychologist at the beginning of the film sensed there was a problem but held his tongue and instead gently suggested they allow their youngest daughter have more social contacts (hence the party). After the death of their youngest sister from suicide it would have made much more sense for the girls to start conspiring against the parents out of resentment. And then after the forcible lock down (which was awful. What the **** is wrong with those parents?) should have been the opportunity for the girls to make their move.