What is the ruling regarding a wife's brother who verbally abuses his sister because he believes she envies him for not having children, and how should she deal with him, knowing that he accuses others of envy?

The gist of the Fatwa:

Children are a gift from Allah, so be patient and supplicate for their righteousness, and do not despair. As for the tribulation of harm from a relative and their ill-suspicion, you must be patient and show kindness to them and their children. Do not worry about exonerating yourself, for Allah knows what is in your heart. And know that your kindness towards them will disgrace them and place them in a position of shame. This tribulation carries a great reward, and remember that the Messengers, Prophets, and their allies were afflicted by the harm of people.