No Joke -- Dane Cook's Brother Steals Him Blind

Talk about keeping it in the family -- sources tell TMZ Dane Cook's brother (who was also his business manager) has been arrested for allegedly stealing millions from the comedian.

Darryl McCauley -- who was in charge of Cook's financial affairs since the 90s -- was arrested today by the Massachusetts State Police and charged with three counts of larceny and forgery.

Authorities say in one case, Dane's bro forged a $3 million check and deposited it in his account.