What is our responsibility as in-laws?
God’s plan for marriage is for a man to be “joined to his wife, and they shall become one flesh” (Genesis 2:24). What your son-in-law is asking is for you to become an intrusion into that flesh.
In a body, if one part is wounded other parts sacrifice to help it heal. If the couple’s problems have progressed this far, their marriage is severely wounded. If your son-in-law is serious about his family, he needs to stop looking to you to solve his problem, and instead take the lead himself. Begin the healing process by forgiving and asking for forgiveness. Once the lines of communication are open and both parties are committed to yielding their own desires in preference one for another, disagreements can be resolved. The goal is to become one flesh—physically, emotionally, and spiritually.
Intrusive parents are often divisive and crippling to their children’s marriages. Please don’t take that road. Your best course of action is prayerfully encourage both parties without taking sides, and demonstrate godly principles in your own marriage.