Caitlin's playing rough with Mark.
My thoughts... This 13 lb girl is "playing rough" with her 22 lb brother! How can this be? But, there she was, clear as daylight, attacking her older brother who was trying hard to play with his cars in peace.
Perhaps this is a sign of things to come... oh boy!
To Mark's credit, he is a caring and loving big brother, who lets his little sister torture him without returning the favor.