This sounds so familiar! It can be so frustrating to you and him both when a close family member does not understand or accept the diagnosis and limitations. I have been in this situation many times with my DS 3, a SPD seeker. I get especially on edge if I know DS2 is sleeping and I am trying to prevent a meltdown from waking little brother up. It can be difficult with another family member is interacting with him because you want them to have their own dynamic, but you also should feel like you can step in when you see DS is starting to meltdown even if it causes friction with your mom. I have gotten into major arguments with my DH and my own mother over this, and the ways I respond to DS. Have you shown you mother any information on SPD? Would she be receptive to it? Sensational Kids has a checklist that is easy to read for subtypes that may explain more about your child, and why you do what you do to your mother. She should respect your reason for stopping the interaction and method of calming DS, even if she does not agree because YOU are his parent. Polite, but firm can be difficult to achieve! Then again, even if you try to explain, she may not be open to it (my DH is not, and my mother does not understand).