Denis' time with his mother's stepsister started 20 years ago in the original story entitled The Hots for Aunt Pat. It began in a hotel room when his Aunt Pat was 50 years old and he was just 30.
He had the pleasure of making love to her when she was in her prime, and for many years after when she wasn't quite as perfect as she once was.
So, as his occasional lover turned 70 years of age, Aunt Pat's adopted daughter reached 50, the same age as her mother the first time she and Denis had sex.
The daughter was also 50 when she and Denis had sex. It wasn't planned with Tracy either. It just happened like it did with her mother 20 years earlier.