Cooper DeJean's brother, Beckett, allegedly had a BAC well above the legal limit when he was arrested for operating while intoxicated over the weekend -- cops say he blew a .142 on a breath test.
TMZ Sports has obtained 20-year-old DeJean's arrest affidavit ... with cops stating the former South Dakota defensive back was "unable to maintain lane" as he was driving his Chevy Malibu around 11:55 PM on Saturday. Once he was pulled over, the officer claimed "a strong odor of alcohol.