Taboo III (1984)
Barbara Scott's son Paul moves out unexpectedly, leaving her distraught. She soon becomes jealous of her son Jimmy's girlfriend while she secretly yearns for him. She confides this to her friend Joyce McBride, whom she discovers is having an affair with her own son, Brian. Jimmy and Brian are both helping prepare a fledgling band for a major tour and record deal, and accomplish the latter by throwing an orgy for the record company executive. After a hot night of passion with Joyce, Jimmy decides to give in and seduce his mother.
While Barbara is ashamed of having had sex with her son, Joyce has completely accepted her-own incestuous side. When Barbara starts craving for her other son, she visits Joyce to seek advice only to find her in bed with her-own other son.