If underneath this problem lies a “transferential anger” that you are bearing due to the anger this couple or your daughter-in-law has toward her parents, perhaps understanding this matter will help you approach this problem with more tolerance. This may be coupled with your daughter-in-law’s deep jealousy over the fact that she did not merit such amazing parents as her husband has merited.
These ideas will hopefully help create less pain and anger in your heart. I realize that it may be easy for me to analyze and explain this situation, but you are the ones experiencing the deep pain in this difficult situation. Hatzlachah in dealing with your challenging predicament – and remember to try to keep your cool at all costs!