unfortunately you can not do much about changing your sister's mind.
We found out that DS when he was 3.5 was very cleverly messing with the other kids behind the teacher's back due to boredom (we had someone observe his room)- the teacher thought he was so well behaved. That was just at 3.5, and no one noticed... it makes me suspect that if he remained in that type of environment where he was so bored, it would manifest in similar goofing off or some other not so healthy behavior that would get blamed as being immature. We got him out of there, and now he is too busy and engaged to come up with stuff like that.
