Elon Musk and his cousin, Lyndon Rive, have always been close. “We’ve known each other for as long as we’ve been conscious,” said Mr. Musk, speaking at a panel this week at a private conference in New York. Mr. Musk says Mr. Rive “is an awesome guy and really hardworking and driven, and you can trust him with anything.” Mr. Rive recounts the drive to Burning Man in 2004 when Mr. Musk told him his next venture should be in solar power—and Mr. Rive says that when Mr. Musk tells you what area to get into next, you get into it. The companies the two men run—Tesla Motors Inc.