What is the ruling regarding a father and mother knowing that one of their children molested his sister and they did nothing?

Harassment of a sister is a heinous evil and an ugly act. The father or brother must prevent its recurrence by eliminating its causes and by explaining the ugliness of the act to the son and warning him against it, for it is a betrayal of trust, as every guardian is responsible for his flock. A Muslim must teach his family the affairs of their religion and forbid them from what is unlawful for them. If the son repents, there is no harm in keeping silent, as it is more appropriate and better for him.