How is Oliver protective of his siblings in the novel, The Vanderbeekers of 141st Street?

Oliver is the only male Vandebeeker child. He often plays with his friends at the basketball courts or with walkie talkies. Oliver often comforts and tries to protect his younger sisters. When they are emotionally hurt by Mr. Beiderman's actions, he writes a letter to Mr. Beiderman to chastise him.