not popular why ?

I don't think so.
"Tendres Cousines" is also French and certainly is a lot more popular.
I saw this one last night in Italian dubbing.
The novel is very erotic, but they couldn't do all the explicit stuff in the film unless they made an XXX movie, and that would be impossible since the main character is supposed to be 16 years old, and in the book he has sex with his two sisters, his aunt and with five servants, and sires three children of which two are incestuous.
Today they wouldn't be able to make this movie at all, for sure. Just the scene in which the boy comes out of the river totally naked and buries his face in the woman's naked crotch...