What should we do with my sister who is committing grave sins such as engaging in illicit relationships, abortion, theft, and disobeying her parents, and is not affected by advice, which causes us constant sorrow and fear?

Your sister's reprehensible actions require admonition and change. You should continue to advise her and pray for her, and try to distance her from bad company and find righteous companions for her. Gentleness and kindness can achieve what violence and severity cannot. You must beware of declaring her an apostate (takfir) or intending to kill her, for the sanctity of a Muslim's blood is immense, and the implementation of punishments (hudud) is the prerogative of the Imam or his deputy.