Donald Trump’s niece applauded Vice President Kamala Harris for doing exactly what she needed to do at the debate in Philadelphia — rattling her uncle early and causing him to “unravel.” Mary Trump joined Anderson Cooper on his show Wednesday night and called her uncle’s performance “incredibly validating,” noting that getting under Trump’s skin is the “way to go” but that no one has been able to do so — until now. “That was the thing that she did so brilliantly,” said Ms. Trump. “Vice President Harris gave both substantive answers to questions to help the American people get to know her bette…