What is the ruling on shunning a brother-in-law who caused family problems and relayed personal disputes, affecting the relationship with the wife's family, especially since there is another marital relationship between the two sisters?

Allah Almighty has forbidden discord and abandonment among Muslims. If there is real harm in associating with the wife's brother, then there is no blame in boycotting him, as scholars have unanimously agreed on the permissibility of boycotting for more than three [days] if interaction brings about a deficiency in religion, or harm to oneself or one's worldly affairs. However, it is not permissible to inflict any harm upon him, or to speak about his flaws in his absence, except in specific situations such as seeking redress or advice. Responding to evil with good brings affection and protects against the evil insinuations of Satan.