Scarlets and Wales flanker Aaron Shingler was tackled without the ball by flyhalf Steven in the 69th minute and was incredulous when referee Lloyd Linton pulled out a yellow card.
As younger brother Steven nonchalantly strolled away from the scene, Aaron trudged off shaking his head as away fans jeered.
BBC commentator Gareth Charles described the yellow card as "one of the most ridiculous decisions you'll see", while former Wales flyhalf Jonathan Davies said it was "absolutely crazy".
Although reduced to 14 men for most of the remainder of the match, the Scarlets held on for a bonus-point win to leapfrog the Blues into sixth place in the competition.
"Clearly he was tackled without the ball and the wrong player was sin-binned," Scarlets coach Wayne Pivac told British media.
(Writing by Ian Ransom; Editing by Nick Mulvenney)
