Exactly what lilolme said. That's what I was thinking as I read your post. My son is younger than yours, but he makes up 'expectations' and 'rules' like that in his own head all the time. Basically, if he thinks something is supposed to be done a certain way--either because it usually is or because of his own sense of logic--it is very upsetting to him when someone does something different. I am guessing your son has deducted on his own that the potholder is only for ovens--unspoken and internal rule--so he got very upset when he saw his sister doing something he thought shouldn't be done.
My son is also very insistent on enforcing the rules we do have. Unfortunately, we are not a super structured family (we're working on some of that), and it probably drives him crazy. We do stick by rules, but we allow exceptions sometimes.
My son has thrown a fit over his sister not getting into the car through the right door (she got in 'his door' and climbed across to her seat. He wailed over it for five minutes, at which point I finally told him he can't control what his sister does. He said--'well, she's not supposed to do that.' And I told him it wasn't one of our family rules. He shouted back, "Well, it's MY rule!!" He still pouted the entire way to school. I can't tell you how much it helped my own level of frustration just to know he wasn't trying to be difficult, it really bothered him and the way he thought.
I bet if a number of parents shared some of the little things we get tantrums over, it may be reassuring to you to know our kids just share a similar way of thinking.