I wrote Polly's Garden for my sister in her final few weeks. When we were children, she would tell me the most fantastic stories. It then stood to reason that in her last months, the two of us would visit the Garden each evening before bedtime, lying in the dark while spinning tales and giggling. It was at once the most heartbreaking and transformational period of my life. I learned so many incredible life lessons from my sister Polly, but...