FBI agents probing the murder of a Staten Island mom in Turkey said they don’t believe she was there just to take pictures — and said she hung out with a “criminal element” during the solo trip abroad. “There are a lot of open questions that need to be answered,” a source familiar with the ongoing case told The Post. Mother of two Sarai Sierra, 33, was found dead from a ferocious blow to the skull in a seedy area of Istanbul last Saturday after failing to show up for a Jan. 21 flight home to New York. “The first...