Diane Keaton opens up about her brother's struggle with alcoholism in new memoir

For five decades, Diane Keaton graced the silver screen as the star of films that resonate with fans of all ages, including “Annie Hall,” “The First Wives Club” and “Book Club.”

Now, the Oscar-winning actress is looking back at her life and career in a new memoir, “Brother and Sister,” a searing account of her complicated relationship with her brother, Randy, and his struggles with mental illness and alcohol.

“When I look back on Randy, I just think, wow -- I wish I could’ve been a better sister," Keaton told ABC News' Juju Chang in an interview that aired Wednesday on "Good Morning America."

In her book, Keaton reveals that as her acting career was taking off, her brother faced a downward spiral. In writing the book, she used journal entries and letters between her family members to better understand what he was going through.

“I went off and I went to acting school. He didn’t have a direction, he didn’t want to go to school,” Keaton said about the period where she first felt that something was wrong with her brother. “It was sort of obvious that it was becoming unusual. I think that Randy was drinking a lot by the time he was 22, 23.”

At the time that Keaton started noticing signs about her brother, discussing the topic of mental health was not a thing. And while he did seek help from psychiatrists, Keaton said that “nothing made a difference.”

Her brother, who is now in his seventies, battles dementia -- which is something that Keaton said has actually brought them closer.

Keaton has described Randy as the single male relationship that was the most intimate in her life.

“I’m a person who’s never married and I think that Randy had the most significance," Keaton said. "And it’s being played out now more than ever, now that he’s -- you know, he’s kind of infirm in a way that he can’t really express himself too much anymore."

Now Keaton hopes that her book serves as a reminder for everyone to treasure loved ones while there’s still time.

“I hope that in some way they take a second look at the people, their family members," she said. "Because, look, I feel like I’m so lucky to have my brother in my life at this point, as opposed to when I was younger when I was so consumed by my life and I kinda lost touch with Randy … the real Randy."

Keaton's memoir "Brother and Sister" is available now.