How can the situation of the sister who goes out with young men be rectified, and what should the questioner do after the mother learned of his molestation of his sister seven years ago?

Repentance from sin is great, and harassing a sister is a heinous act. Were it not for Allah's grace, worse would have happened. If the sister goes out to meet young men, her guardian must advise her and prevent her from going out if necessary, because she is a trust and he is responsible for her. Children are trusts with their parents, who must educate them, discipline them, and protect them. If the sister disclosed what happened, she has done wrong. If the mother asks, use allusions (evasive language) or deny the matter to avert greater mischiefs.