Peter and Wendy
Peter Pan ran away from his parents when he was a baby as told in "Peter Pan in Kensington Gardens" and "Peter and Wendy". Finding the window closed and seeing a new baby boy in the house when he returned some time later, he believed his parents no longer wanted him and never came back. This younger sibling is referred to in the chapter "Lock-Out Time" in "Peter Pan in Kensington Gardens" but is not mentioned again.