The two-time Oscar winner is one of the most acclaimed actors of his generation but Tamasin Day-Lewis insists he battles feelings of self-doubt with every role he takes on.
She tells Britain's ES Magazine, "Look at the work. That's it. You can't teach it, bottle it, explain it. So much of it is sheer hard graft. When people imagine that Dan's doubt as an artist must have vanished by now, they couldn't be more wrong. It all gets more difficult. The stakes get higher."
Tamasin, a TV chef and documentary maker, also admits she and her little brother were not especially close to their parents, poet Cecil Day-Lewis and actress Jill Balcon.
She adds, "We were mostly left alone. We didn't holiday with our parents until we were nine and six. We were sent instead to our grandparents in Sussex (in England), or to the Black Mill House Hotel at (nearby) Bognor Regis with nanny. (At home) we didn't come down for dinner, we had tea in the nursery with nanny, and were thrown together into a solitary world... which led us straight into the landscape of the imagination."
Tamasin also recalls lashing out against her strict upbringing when the siblings were sent to boarding school: "We both did rebel most of the time. Drinking, smoking, stealing, organising a hunger strike, escaping late at night to the opposite sex's dorms, we led by bad example. The strict Victorian upbringing we went home to... meant I specialised in breaking rules. I egged Dan on and, he has told me since, was an evil influence. He insists I made him steal for me. My defence: it was his choice. We were partners in crime."