As Charlie's sister, Sara was really worried about Charlie's missing. I read the book chapter 11, on page 58, she found Charlie was not in the house, she told her aunt and statred finding anxiously. If I'm Sara, I will be so careful about Charlie, because he can't speak and sometimes he's abnormal.
Sara and Charlie were always stay together, and Sara helped Charlie a lot, maybe it mada Charlie doesn't have good self-understanding, and he was gravititated by the swans, he wanted to have that freedom, he wanted to be like swans. He waned to find back his self-acceptance because he's an amentia.