The Scarlet Ibis

From the text:

It was bad enough having an invalid brother, but having one who possibly was not all there was unbearable, so I began to make plans to kill him by smothering him with a pillow. However, one afternoon as I watched him, my head poked between the iron posts of the foot of the bed, he looked straight at me and grinned. I skipped through the rooms, down the echoing halls, shouting, “Mama, he smiled. He’s all there! He’s all there!” and he was.