A letter signed by the four Hinduja brothers is at the center of a legal dispute over the future of the family's $11.2 billion fortune. The 2014 document says that the assets held by one brother belong to all, and that each man will appoint the others as their executors. But now Srichand Hinduja, 84, the patriarch of the family, and his daughter, Vinoo, want the letter declared worthless.