I am the step mother to an 11 year old boy.  My boyfriend has 50/50 custody.  At our house, SC is an only child and at his mothers house, he has older and younger siblings.  At his mothers house there aren't many rules, but i have observed SC acts very different around his younger sibling.  Very mature and calm.  At our house he's bouncing off the walls, rude, argumentative, and basically a jerk.  He needs so much attention which doesn't make sense because he has no other kids to compete with.  There isn't just one behavior that is making me loose it, but collectively as a whole.