Dear Son-in-Law,
If you have a good relationship with your father-in-law, engage him in conversation about his thoughts and feelings about his behaviour and relationship with his family members. Do so from a place of curiosity rather than judgement. If you sense openness or a hint of a desire for change, go deeper to explore with him how to do so. If he is defensive or closed, you will need to use indirect methods.
Firstly, if you attend the same church, enlist the help of church friends that he respects and would be open to listening to. This may need the agreement of other family members attending the same church due to the sensitivities of revealing his home life to others.
Secondly, model respectful behaviour in your own interactions with your wife and children. It may be that your father-in-law is repeating family patterns that he himself was exposed to. Your own example signposts a different way.
Thirdly, help your father-in-law improve his mood—and the overall culture of the home —by engaging him in activities that you know he enjoys. Research shows that doing so improves one’s mood more than doing nothing or suppressing the bad mood.
Fourthly, pray! Only God can change a man’s heart. Pray for God to intervene and ask other family members to pray along with you. This may also lead them to be open to hope for change and be willing to join you in influencing him to do so.