My 11 year old stepson pretends to stab his 16 month old half sister. He also throws pillows at her from across the room, an I also catch him grabbing her head and shaking it back and forth. He also likes to play to rough with my 5 month old. We only get him for 2 months in the summer and I don't want to be to harsh on him. I just want to know if this is somewhat normal behavior for a 11 year old.