Order is first known legal summons issued to a member of US president's family in the ongoing probe of meddling in 2016 election

Move comes after US president invokes executive privilege to block lawmakers from full Mueller report on Russian interference in the 2016 election

Decision comes as House Judiciary Committee meets to consider holding attorney general in contempt of Congress over failure to release report

US president insists Mueller's report clears him, says Democrats' efforts to investigate him will prove politically costly

Jerrold Nadler, who heads House Committee that would hold impeachment proceedings, says up to Congress to investigate obstruction allegations in Mueller report

But Pelosi has resisted calls to launch proceedings to oust Tump, insisting on a methodical, step-by-step approach to the House's oversight

Special counsel's probe shows how Trump tried to clamp down on investigation, but with no criminal charges, it's unclear if Democrats will pursue impeachment