Cruel Intentions 2 (2001)

Plot Outline:

Takes place several years before the malicious movie, back to when Kathryn first meets Sebastian, their sexual attraction to each other and their enjoyment in destroying the lives of their peers. A few years before the events depicted in the 1999 film. It follows the lives of rich step siblings Sebastian Valmont and Kathryn Merteuil as they playfully manipulate the people surrounding them at their exclusive New York prep school.

See:
# Cruel Intentions 2 at Wikipedia