There’s some lesbian incest in this exploitation film from the UK.
The sophisticated plot was probably stolen from Tolstoy:
A young man fantasizes about fornicating with several women.
Not long after they made this film, Madeleine and Mary Collinson became Playboy’s first twin Playmates in the October, 1970 issue. By then, they had curly, dark brown hair (probably wigs).
They followed that with appearances in two 1971 films:
The Love Machine
Twins of Evil – in which only Mary did nudity!
They also posed nude for various “lad’s mags”:
Just Madeleine
both
To my knowledge, neither of them acted after 1972. Mary raised a family in Italy. Madeleine raised three children in the UK, and later moved to Malta, where the twins were born. They have both left us. Madeleine died in 2014 from emphysema complications, and Mary passed away in 2021 from broncho-pneumonia.
* Here are the videos from The Love Machine and Twins of Evil

You’ve got things the wrong way around.
The twins’ natural hair is brown, and it was Madeleine who appeared naked in ‘Twins of Evil’ (although both of them posed nude for publicity shoots).
Yes, Some Like it Sexy was original to Tolstoy, while The Love Machine is clearly a lift from Theodore Dreiser.
According to IMDB the Collinson twins were born in Malta on 22 July 1952.
“Some Like it Sexy” was released in August 1969.
The twins would have only celebrated their seventeenth birthday a month before release making them sixteen when the film was made.