Facilitating the Negative Effects of Their Sister
Because of a different reason, this person has severed ties with their parents. By allowing her daughter to act out in ways that are detrimental to her well-being, her parents have done more harm than good to their own family. The sister of the original poster has shattered the family with her manipulative and mean-spirited behaviour.

Their parents, on the other hand, refuse to do anything about their daughter’s toxic actions. In order to raise their son, the OP has had no choice but to do so without the influence of their own family. Isnt it a dilemma to select between a family or mental peace.