What is the ruling of Islamic law regarding a man who harassed his brother's wife with WhatsApp messages, exploiting her husband's absence, and what is the legitimate Islamic way to deal with this situation to avoid scandal and achieve reconciliation between them?

The husband must defend his wife against anyone who assaults her, and he must balance his actions between achieving benefits and minimizing harms. Killing a brother is a great harm with no benefit, because achieving the benefit of preserving one's honor does not necessitate killing, severing kinship ties, exposing oneself to imprisonment, and the dissolution of the family.

To protect his wife, he must change her phone number and prevent her brother from contacting her. He should instruct her not to open the door for him in his absence. He should move from this house to one far from his brother, or change his job to a daytime one.

If changing residence takes time, the husband should not leave his wife alone in his absence. Instead, he should leave someone from his family or her family with her to repel harm. The most important step is that the husband should not allow his brother any opportunity to reach his wife, and he and his family should stay away from temptations.