The Stepsister(1997)

In this family, murder is relative.

The Stepsister is a psychological thriller about a college student majoring in psychology who must use her intelligence and instincts to uncover the sinister plot of her father's new wife and stepdaughter. When the newlyweds arrive, the psych major grows suspicious of their true motives, sensing something dangerous lurking beneath their charming facade. As she digs deeper, she discovers that the two women are far more calculating and deadly than anyone suspected. Trapped in a house with predators, she must outsmart them before they strike. The film builds tension through psychological games and hidden agendas, as the young woman races against time to expose the truth and protect her family. With her education in human behavior as her only real weapon, she becomes the family's last line of defense against a carefully orchestrated threat.