What is the ruling of Islamic law on dealing with a sister who mistreats the wife and causes family problems that may lead to divorce, and how can this problem be solved religiously?

If the sister provokes the wife and wrongs her without cause, then she is an offender and a troublemaker. What is required by Islamic law is affection and love among Muslims, especially among relatives. The Prophet, peace and blessings be upon him, said: "Do not hate one another, do not envy one another, do not turn away from one another, and be, O servants of Allah, brothers."

She should be advised gently, and if she persists, she may be boycotted if that will deter her. The opportunity must be denied to Satan, who seeks to cause discord between spouses. If the wife lives with the in-laws, that may be a cause of problems, and the wife has the right to a separate dwelling. The husband must appease his wife and treat her kindly to avoid divorce. If he only visits his sister on occasions to avoid her harm, then you have been spared her evil.