Hillary Clinton's brother-in-law is sitting in a jail cell after getting busted for DUI in Southern California ... just 2 days before the crucial CA primary vote.

Law enforcement sources tell TMZ ... Roger Clinton was arrested Sunday just after 8 PM in Redondo Beach. He was booked for driving under the influence, and we're told he refused blood alcohol testing.

Roger remains in police custody. His bail is set at $15,000.