Dory (1990)

In an isolated prairie farmhouse, on a sweltering summer night, two sisters are locked in a battle of wills. Tension, frustration, anger and hatred seethe between Robin and Dory as the mystery behind their struggle deepens. At first, Robin seems in control of the fragile Dory, but Dory's weakness is an illusion and gradually Robin's physical domination surrenders under Dory's barrage of cruel mind-games as the confrontation slowly descends into a night of madness.