Diane Keaton hasn’t lived a traditional life (an Oscar-winning actress, she never wanted to be a wife, she says), but family remains the center of her world. And family includes her younger brother, Randy Hall, who she says suffers from dementia.

“I’ve spent a lot of time in senior living facilities [recently] with my brother, Randy, who has dementia,” the 73-year-old actress—who’s now starring in her 58th film, Poms–tells People in a wide-ranging interview for this week’s issue. “I’m writing a book about him. He always had mental issues. Nobody could figure it out,...