What is the ruling on ridiculing a sick girl—who has now passed away—and what should we do to expiate this sin?

Mocking a Muslim and ridiculing them is forbidden in Islamic law. The two sisters must repent and seek forgiveness for what they have done. If their repentance is sincere, the effect of the sin will be erased. Since the girl has passed away, they must strive to seek forgiveness and supplicate for her and her family for good.