Taboo II (1982)

a XXX all time favorite classic

from imdb
the second part of the taboo concerns the family of Sherry McBride, portrayed by Dorothy Le May who apparently played a girlfriend character in the original Taboo. (This film, incidentally, was Le May's last adult film, though some she'd made earlier were released after it.)

While there are some touches of realism -- Sherry initially reacts with horror and disgust to the news that her old boyfriend has been having sex with his own mother -- they are of course subverted by the need to have as much sex in the film as possible. (A bit of dialogue between Sherry and Barbara, indicating that she's still not happy that the other woman stole her boyfriend, even though she's willing to have sex with her, might have improved matters.) Another nice touch is the lengthy, increasingly inebriated monologue that Honey Wilder's character, the mother of incestuous siblings Junior and Sherry, gives after discovering what they're up to, in which she gradually talks herself into having sex with her son.