Oh Diana.. Please don't get upset , your MIL and her family sound like ignorant people. I am sorry to say that. You are a wonderful woman , she should be so happy that her son has a wife who has been by his side for so many years and have mutual love and respect for each other. Not to many couples have what you have in your marriage. I think she sounds like a bitter old woman, and your SIL are no better. They probably are all jealous bitties.
In all honesty your husband shouldn't have told you . I understand that he is hurt also but this will sever whatever little relationship you have had. As far as your kids, what kind of grandmother would favor one over the other. My mother had 10 and loved them all equally. Diana honey these are your children and to h*@**@ with her..
I will keep you in my prayers. Diana it's not you, you are a beautiful person inside and out.. We all love you so please don't cry anymore...