Siblings

We have 14 yrs between our oldest and youngest with several children in between. So we regularly rely on our teenagers to watch younger children when we're not home and also help out when we are. Our oldest son is 15 now and is frequently frustrated with his younger siblings and tends to boss them around and try to control them. I'm not sure how much authority to grant him for when he's in charge, and how to teach him not to be overly bossy, especially when parents are around.