How does doodle disappoint his brother

From the text:

He was born when I was six and was, from the outset, a disappointment. He seemed all head, with a tiny body which was red and shriveled like an old man's. Everybody thought he was going to die-everybody except Aunt Nicey, who had delivered him.

I wanted a brother. But Mama, crying, told me that even if William Armstrong lived, he would never do these things with me. He might not, she sobbed, even be "all there." He might, as long as he lived, lie on the rubber sheet in the center of the bed in the front bedroom where the white marquisette4 curtains billowed out in the afternoon sea breeze, rustling like palmetto fronds.5 It was bad enough having an invalid6 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.