How do you learn about Allie ?

In chapter 5, Ackley remains in Holden’s room, telling them about a girl he had sex with, but Holden knows that he is lying, for whenever he tells that same story, the details always change. Holden tells him to leave so that he can write Stradlater’s composition. He writes about his brother Allie’s baseball mitt. Allie, born two years after Holden, died of leukemia in 1946. The night that Allie died, Holden broke all of the windows in his garage with his fist.