M y mother gave me the best gift imaginable – three sensational sisters . Sisters possess a psychological shorthand; instinctively understanding each other. When one of us is going through a rough time, the other three rally with offers of chicken soup and/or internal organs. If under attack, the wagons circle. Although… wagons? Who am I kidding? Having three formidable sisters on your side is like having a bombproof, flame-retardant armoured vehicle on hand for quick getaways. Of course, it’s not the same for all sisters. In fact, some sisters have enough chips on their shoulders to open a casino. Olivia de Havilland and her sister Joan Fontaine spent their lives circling each other like Cold War spies. When Fontaine was awarded the Oscar that de Havilland expected to win, it was no-talkies for a decade. I was lucky enough to be friends with one of the world’s most famous sisters, Jessica Mitford .…