Maybe this;
A single female psychiatrist in her mid 40's get a visit from an upper class woman who tells the psychiatrist about her incestuous relationship with her son. After she tells her detailed story about the seduction and fucking with her son, the client leaves.
When the psychiatrist is home and in bed that evening, her mind starts to overflow with fantasies about herself and her own son. The client's story has turned the psychiatrist on and she can't stop thinking about it.
Until finally one evening when she seduces her own inexperienced virgin son and they spend all night and weekend in bed.