J.B.
I think if she were throwing a fit over something that can't easily be accommodated, like she won't have them there because of a tattoo or piercing or hair style, or if she made up some ridiculous rule that the world had ever heard of and wasn't the accepted norm, you'd have a leg to stand on. But hats? It is rude to wear them indoors. It's not something I really notice when we're at home, but my parents notice and will remind my sons to take them off at their house (they range in age from 13-21) and they quickly apologize and comply. Tell your sons to grow up and take off their hats indoors at you mom's house, period. It is asking very little of your sons to respect the wishes of someone who should matter to them. My kids know the dress code/rules for when we go to my parents' house...no hats indoors, no clothing that is super tight or shows a ton of skin, no t-shirts that mock things that they value, etc. They can put someone else's values and wishes ahead of their own for a few hours at at time. It's really not asking too much.