God has blessed me with wonderful in-laws. We have never had a problem. This is not true for many. Through the years many have come to me for advise on how to cope with their in-laws. What can you do when your in-laws treat you in such a way that you want to be anything but Christian toward them? Instead of refusing to ever see them again, you can work at making your relationship civil in several ways.
Give it time. For some reason they are not accepting you, but you need not return a like attitude. Time is a great healer.
Focus on what you have in common, not your differences. Do you agree on politics, food, cars, religion? Augment the things you agree on.
Allow them space, both physical and emotional. This is especially important if you also happen to live with them.
Respect their personhood as well as their possessions.
Treat your spouse (their son or daughter) with love and respect. That will be the best bridge to strengthening your relationship with them.
Pray that God will give both you and them the proper attitude.