They had been playing alongside each other happily until the little one shrieked. Roo had done something that upset her. The shriek is her way of communicating her annoyance. Unfortunately Tisha's shriek really bothers her brother, the noise hurts his ears. He grunts and flaps his arms about, then carries on with whatever he was doing. Roo is still upsetting his sister, so she shrieks again. That is as much as the poor boy can handle, so he retaliates by hitting out, kicking or biting depending on where she is in relation to him. Tisha is old enough to fight back so if mum isn't quick enough it turns into a full scale war with more shrieking and more loss of control, bruises and tears.
One day I will be able to get my son to understand that when his little sister shrieks he simply needs to stop. One day I'll be able to go to the bathroom without a war breaking out down stairs.