Bernardo and Homolka swiftly embarked on a sadomasochistic partnership, with Bernardo assuming the role of a cruel master and Homolka willingly submitting as his slave. Throughout their relationship, Bernardo engaged in savage acts of rape against innocent girls in Scarborough, all the while with Homolka's full awareness and consent.

Bernardo and Homolka eventually became betrothed. Homolka confided in a friend, expressing how content she was with their relationship, stating that Paul was exceptionally loving and romantic. However, unbeknownst to her, Bernardo had grown weary of their union after three years. He voiced his dissatisfaction to Homolka, citing her lack of virginity when they first met. Consequently, he redirected his twisted desires towards Homolka's 15-year-old sister, Tammy.

Rather than being appalled by Bernardo's disturbing inclinations, Homolka once again encouraged them. She informed Bernardo that she wanted him to take her sister's virginity as a Christmas gift. Thus, the heinous crime that remains one of the most unsettling acts committed by the Ken and Barbie Killers was set into motion.

On December 23, 1990, during a Christmas gathering at the Homolka family residence, Homolka drugged her sister's beverages with stolen animal anesthetics from her workplace. That night, while the rest of the family slept and Tammy lay unconscious, Homolka placed a cloth soaked in Halothane over her sister's mouth. Together with her fiancé, she proceeded to sexually assault Tammy, capturing the entire brutal incident on video.