I like being a twin. It defines who I am,” Lavinia Osbourne tells me on the 49th birthday she shares with her sister, Michelle. “It’s amazing to have a twin and have a built-in friend for ever,” Michelle says. “I’ve been really blessed to go through this journey with someone else.” Lavinia and Michelle know that those of us who haven’t shared a womb with a sibling can be fascinated by twins: their similarities, how they differ, whether there’s any kind of mysterious synergy between them. “There’s twin magic. It does exist – it’s a thing,” Michelle says. “I can feel when she’s upset, and she can feel when I’m upset.” They have even felt each other’s physical pain, Lavinia says. “There was a time when she spilled hot water on her leg, and I felt it.” Lavinia and Michelle aren’t identical twins. They share the same striking eyes, but the lower halves of their faces are different.…