Is there a way to atone for the sin of harming a girl ten years ago, who then died along with her fiancé, and is the daughter's current illness a punishment for that sin?

This highlights the grave danger of transgressing against and oppressing others. The transgressor must repent, seek forgiveness for the wronged person, supplicate for them, speak well of them, and dedicate the reward of righteous deeds to them. This is so that Allah may forgive the transgressor and alleviate the suffering of the wronged one. The illness of the daughter should be treated through legitimate means such as supplication (du'a), charity (sadaqah (definition)), ruqyah (definition) (spiritual healing), and medical treatment. The supplication of a mother is answered, and charity cures illness.