Mother-in-Law treats my child different?

Ok so my boyfriend and I have been together for about a year and a half, we've been living together for about six months now. He has a almost 3 year old from a previous relationship and I have a 20 month old son also from a previous relationship. He and I have been together since my son was 5 months, so my boyfriend is "daddy." His mother is the sweetest woman, she has always been good to my son and I but when it comes to the boys it is obvious she favors her grandson. I completely understand her, she's helped raise the boy and there is no way she will ever feel that love for a child who is not biologically her grandchild and thats fine. However, I do feel that at least when she is around both the boys at the same time she should treat them equally. My family, mostly my mother is the same. My does she absolutely love my son, and she spoils him rotten, but when we've gone over and we have both of the boys she will not favor my son. If my son asks for a popsicle, she will bring one for the other boy too, if my son snatched a toy from my stepson my mother will snatch it and give it back, on christmas she did my more gifts for my son, but she kept them at her house and allowed him to open them when he was there alone. She bought both the boys a gift of equal value for them to take home. On Easter, she made all of her grandchildren baskets including my stepson. She asked me what he needs for his bday because she'd like to buy him a gift. My son wants to play outside, she invites the stepson. She does these things because he is part of the family now. Now my mother in law, will buy things for her grandson and give it to him right infront of my child, my son stares and reaches his little hand to have his stepbrother pull back and say "no! mine!" She takes him for walks, never invites my little boy. She does these things all of the time. It bothers me because she could at least include my son when he's around, the boys are not always together but she always excludes him. What should i do?