Diane Keaton's brother is suffering with dementia and doesn't know who she is.

The 73-year-old actress is planning to ''write a book'' about 71-year-old Randy Hall because, prior to developing the debilitating condition, he lead a ''very interesting'' life, although it was quite tough.

She told People magazine: ''I've spent a lot of time in senior living facilities [recently] with my brother, Randy, who has dementia. I'm writing a book about him. He always had mental issues. Nobody could figure it out, really.

''[He] has had a hard life, you know? He's a very interesting person. Very sensitive, a writer and poet. But also a big drinker, and completely solitary. It's so complicated. I don't know if he knows who I am.''

Before developing dementia, Randy - who can no longer walk or talk - was diagnosed with Parkinson's disease and was supported by his parents, Dorothy and Jack, who have both since passed away.

The 'Poms' star has since taken over looking after her brother and Diane - who also has two younger sisters, Robin, 68, and Dorrie, 66 - visits him every Sunday at his senior facility, and thinks his ward is full of ''fabulous'' people.

She added: ''There are fabulous characters on the dementia ward. When you're there with all of them, it's not like, 'They're really weird.' They're people.''

And the 'Book Club' actress explained that she thought it was ''adorable'' when she noticed two patients holding hands whilst they went on a walk.

She continued: ''It was the most adorable thing.''