From John Fawcett's Ginger Snaps by Ernest Mathijs
Isabelle and Perkins are both on record talking about how Heavenly Creatures; Sister My Sister; and Girl, Interrupted were major points of reference in building the sisters' characterization. Karen Walton also echoes that Heavenly Creatures was an influence on her writing the screenplay.
Sister, My Sister is literally a film that is explicitly about incest, being a fictionalized account of a pair of murderers who were accused of incest.
And Girl, Interrupted... well...
The incestuous element was built into the film from the ground up, deliberately written into the screenplay and elaborated upon as the actresses further developed their characters. It permeates the narrative like the rot of carcasses slaughtered by the Beast of Bailey Downs that everyone normalizes even though it is an ever-present threat - that ultimately takes human lives as well as pets' lives. The codependency, the shadow of incest, the unhealthiness fostered in small towns are all integral elements of the sisters' dynamic. It's one of the major sources of horror in your horror film.
And don't even get me started on the third film, where the director outright says things like this:
But again, even if people don't know about the behind the scenes, how do they hear the lines "We're almost not even related anymore" and "You want me?! You want me?! Then stop hurting everybody else and take me, take me!" and think, "This is normal?"
Like, that's not even subtext at that point, it's just TEXT.