Hello J.: As the mother of 5 children, & a foster parent. I have seen this a few times.
I have always said to my children the old addage-- You, must be a friend to have a friend, or You, will be treated as you treat other people.
With children, you don't get to pick friends for them. I know that even at this age when mine were agressive/controlling, they got left out, as others did not have to put up with it and didn't. I can say that I don't put up with people that try and control my adult life. You are lucky, to have a teacher that is on top of things and can help you have a chance to do things to make changes. We have several teachers in the family, and theydo say that telling a parent something they do not want to hear is not always recieved well.
I saw with our children, that the eldest, was trying to be my helper, and I without realising it had her doing things that gave her more power than she should have had, to help me. She just naturally took it a step farther, into all her areas of life- dance lessons, church,school. It took her crying because she wasn't invited to an activity with the neighbor children to help us rethink our family dynamics. The other child was just so active and curious that it made people crazy! If there was a mountain to be climbed then he was the first to get there-- other children were not this way, so he felt left out and alone. It took again, a child that was generally timid, finally saying that he was scared of the things that our son did to get us to see. (the boys were digging to China, and I planted a tree in the hole).
You must be the ones to say STOP, THINK, CONSIDER OTHERS FEELINGS. When a brother gets upset over it what do you do? What does he do? Is there a consequence that has a price high enough to make her think 2x's before doing it again? We used role reversal as a family activity a few times and learned alot. You may find it worth trying. I know that this is the start of your adventure with outside influence with your children. It will be OK, in the end. I promise that eventually they either grow up and grow out of it with the advice and example from you, or they become lonely. I have 5 wonderful adult children. Everyone has become something exciting for a job choice. The one that was willing to dig to China, is still interested in what goes on and has made it part of his life. The eldest is in the medical field.I have yet others in law enforcement, and firefighters, teachers, and proud stay at home mothers. Good Luck, Nana G