She wrote a memoir about her relationship with her alcoholic brother Randy titled "Brother & Sister", published in 2020. In it, she published a letter that her brother wrote to her after seeing her in one of her films: "There are times in Reds (1981) when I wanted to stop the projector, so the moment wouldn't move so fast. Where did you learn to use your face so well? I think you ran across every emotion in the book, then threw the book away and made up some of your own". When a journalist read the letter back to her, Diane could barely hold back her tears when she responded, "That was sweet of him. I don't know what to say....I'm sorry.".
