I wouldn't argue with her. I also think that if she is treating your youngest son different, than I wouldn't bring him over to her house. I have a grandma something like that. She picks favorites. I have a aunt that she has went way and beyond to do whatever makes her happy. As a result of this, her other daughter can't really stand her, and my mother has passed. My grandma only has one daughter that she is close to, but my aunt treats her bad. One thing that I have learned in my life is that you will also be treated the way you treat others. If your grandma is really upsetting you than just make small talk with her. If she is being nasty to you, try and change the subject. I would tell you to let her be, but your son is over there.