For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
Act I of Brother's memory, the story of his time with Doodle, opens when Doodle is born. It covers the time leading up to Doodle's birthday walk, and includes the time after, when Doodle and Brother spend time together telling stories and enjoying the Beauty of Old Woman Swamp.
Act II
In Act II things get a bit grim. The act opens with Brother's vow to turn Doodle into a running, jumping, boat rowing, and "normal" boy before he starts school in less that one year. It ends when Doodle buries the scarlet ibis in the flower garden that fateful Saturday afternoon, the weekend before school starts.
Act III
This tragic act begins with Doodle's horrific rowing lesson. It includes his abandonment by Brother in the storm, and Brother's horrific discovery that Doodle died in between Brother's running away and his coming back. It ends with the image of Brother covering Doodle's dead body with his own.