HiCustomer Thank you for asking your question on Just Answer. By the way, it would help me to know:
-What are their ages?
-How long have they been married?
-Did you get along before they got married?
-Have you talked to your son about this?-Could you explain your situation a little more?Thank you again for trusting me with your problem. Please reply as soon as possible so that I can finish answering your question.
It almost sounds like his girlfriend is trying to be his mother and control every aspect of the relationship including who he is allowed to see and who he isn't. She seems to be trying to push you (his family) out of the picture so that he has nothing to do with you because you have an influence on him and she wants to be in control of everything. She may have came from a family that acted the same way she does and since her daughters are acting the same way it seems like it was a learned behavior, the girls are learning from their mother how to be rude and uncaring but you shouldn't stoop to her level and that is what you would be doing if you didn't send them gifts and cards for Christmas also.
Your son needs to take a stand as far as the girlfriend is concerned and tell her that her behavior is disrupting his family unity and that she should try more to get along with his family because his family has done nothing to her to warrant this behavior from her. Someone needs to hold her accountable for her actions and the way she treats people. Maybe you should talk to her mother and tell her how she treats you and that you have tried everything maybe she could give you some advice on how to handle her daughters rudeness.
You can't really change her and make her respect you she has to want to do it on your own and since she hasn't shown you an ounce of respect in the 9 years her and your son have been dating then I really don't think she will unless your son puts his foot down and gives her an ultimatum to change her behavior towards you or lose him in the process. Your son may not notice her behavior any more because it's so common place now but it's not fair to you, you have tried everything and still she treats you badly. Maybe you should ask her to lunch and talk to her about it and tell her how you are feeling about the way she treats you and your family and that you are trying and she won't give you an slack at all. Ask her if there is something you to did to her to make her resent you the way that she does maybe if you call her on it face to face she will see her ways and try to change for the better.
Tell her how things are stressing your son out and that you are worry about his health and ask her if you could possibly work together to make things better for everyone involved mainly the children, since she isn't married to your son you have a right as his mother to demand respect from her but it's really up to your son to make her respect you as his mother and the grandmother to their son.