It really irritates me that when i go to my son's father house to spend time with him and my son that his mother always trys to take my son away from me when im holding him, or playing with him, she doesnt just ask she just reaches for him and grabs him, well the last time i went to the house she took my son downstairs without telling me or his father. one time she came with me to the doctors and she was trying to act like my sons mother undressing him for the doctor taking him out of his car seat trying to redress him... ect. like he is not your son he is mine it is so irritating but i dont want to be rude to her because that still is my boyfriend/son's father's mom -___- ughh !!
I think you just need to address this. She is your SO's mom, but you are your son's mom.
Or stop taking your son over there... have your SO come and see you instead.
Just ignoring it isn't going to make the situation any better. You need to explain to her how you feel. Hold your ground. Some of the stuff I could let slide, but communication is the key to a successful relationship with anyone. She might not even realize what she's doing.