What should the questioner do after the death of her disabled sister, who used to be harsh and beat her?

Cruelty and violence are reprehensible in Islam, while gentleness is commendable. Siblings should be characterized by gentleness and mercy, for the elder brother deserves respect, and the younger deserves mercy. If the sister is young and has not reached puberty, striking her is not permissible, because disciplining a minor is the right of the guardian (father, mother, grandfather, guardian), and under conditions that it not be brutal, and not exceed three strikes. Your striking her in this case is an aggression. However, if she is an adult, striking her is permissible to change an evil when it is being committed, if no other means exist, and on condition that there is no fear of discord. As for after the transgression has ended, striking is not permissible because it is a punishment and a disciplinary measure. If your striking was unwarranted, then seek forgiveness, pray for her frequently, and give charity. Repentance is required in all circumstances.