From the beginning pride comes into play, so this quote supports both the plot and the theme. "He was born when I was six and was, from the outset, a disappointment."
One morning when he was making plans to smother Doodle, Brother got a better look at him. "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." This gives some room for the plot. Brother could work with Doodle a little to make him "better."
Brother's frustration with the helpless Doodle became apparent when he wasn't around his parents. "A long list of don'ts went with him, all of which I ignored once we got out of the house." This is the first of many cruel things he does to Doodle.
At the point when he's first trying to teach Doodle to walk, Brother admits the theme of cruelty. "I did not know then that pride is a wonderful, terrible thing, a seed that bears two vines, life and death." The "seed" had been planted, and Brother was set to teach him or else.
The most important quote that ties the story together is at the end when Doodle has died because his brother left him. He and the Scarlet Ibis are described so similarly because the bird symbolizes Doodle. "He had been bleeding from the mouth, and his neck and the front of his shirt were stained a brilliant red." He refers to Doodle as his "fallen scarlet ibis."