Saira Khan, the sister of San Bernardino shooter Syed Farook, told CBS News in an interview broadcast Friday that she could not fathom the possibility that her brother and his wife, Tashfeen Malik, would commit mass murder. Republican presidential candidate Donald Trump says he's not buying it. "I probably don't believe the sister," Trump declared in an interview with "Face the Nation" set to air Sunday. "So you would go after her?" host John Dickerson asked Trump. "I would go after a lot of people and find out whether or not they knew. I'd be able to find out. Cause...