John R. Lausch Jr. set his sights on a career in the law a long time ago, Lausch’s sister said Thursday after her brother was publicly sworn in as the top federal prosecutor in Chicago.“John wanted to be an attorney since he was a young boy,” Amy Mihelich told the friends, relatives and colleagues who packed the ceremonial courtroom in the Dirksen Federal Courthouse for her brother’s investiture as U.S. attorney for the Northern District of Illinois.At least, she added, after he gave up the yellow …