What should a brother do with his sister who talks to a young man on the phone, and he has advised her, but she has not responded, and her parents have not admonished her, and he detests cuckoldry and is protective?

Jealousy is a human instinct that arises when one feels others are sharing in their rights or violating God's sanctities. It is legitimate for the preservation of lineage, and abandoning it is blameworthy, with the one who does so being called a "dayyūth" (pimp or cuckold), which is among the major sins. To solve the problem of the sister who is speaking with a non-mahram man, one must investigate the underlying reasons, such as: emotional emptiness, weak faith, idleness, looking at forbidden things, being in seclusion (khalwa), and enabling access to communication devices. Messages should be directed to the sister, reminding her of the danger of her actions to her reputation and the reputation of her family, and the consequences thereof, such as disliking obedience and hating her family. For treatment, it is recommended to continuously advise her, introduce her to righteous women, motivate the parents to advise her, threaten the corrupt man, expedite her marriage, and supplicate for her guidance.