What is the ruling of Islamic law regarding the extent of responsibility of children - a brother and two sisters - towards caring for their divorced, elderly, and ill parents? And is there sin upon the sister who does not permanently stay with her father, or upon the brother who does not provide the necessary care for his father?

Filial piety is among the best acts of devotion and the greatest acts of obedience, especially when parents are elderly and in need of care. It is an obligation upon all children, male and female, equally, and includes serving and caring for them. Your visits to your father and attending to his affairs are good deeds, and there is no blame upon you if you leave him in a state where you do not fear for his neglect. As for your brother's and sister's neglect in caring for and visiting your father, it is strange and reprehensible. They should be advised gently and urged to seize the opportunity of showing filial piety and caring for their parents to avoid regret.