Marlon's oldest living son, Miko Brando, tells TMZ ... "The Marlon Brando family has heard the recent comments by Quincy Jones and we are disappointed that anyone would make such a wrongful comment about either Marlon Brando or Richard Pryor."

That's in sharp conflict with what Richard's widow, Jennifer Pryor, told us last week ... that the Brando/Pryor hookup DID go down, and Rich was fine with it.

You'll recall, Jennifer said Richard wasn't upset at all by Quincy's comments -- adding the comedian was always very open about his bisexuality, apparently having documented his experiences in diaries he kept.

Miko's gone to bat for his late father before -- most recently when the actor was accused of plotting a real rape during a movie scene. Heck, even Richard's own daughter, Rain Pryor, denied the love affair between the 2 stars in a now-deleted post ... saying Pryor had a relationship with a trans woman once ... but "Daddy did NOT have relations with Brando."

So who's truthin' and who's lyin'?