As the episode revealed, however, Trent Faber was actually a bigamist, and two years after marrying Katherine, he married another woman--Amy--while going under the name, Roger Lansing. During their marriage, Roger and Amy welcomed a son--Heath--making Heath and Brianna half-siblings. Brianna and Heath were unaware of this, even as their relationship ended up resulting in Brianna becoming pregnant with Heath's baby. Once Heath saw Roger/Trent with Brianna, both truths were revealed by both sides. Heath and Brianna revealed their relationship and their eventual parenthood, while Roger/Trent revealed that they were half-siblings due to his bigamy, while adding that they couldn't have the baby and stating that what they were doing was wrong, with Brianna pointing out the hypocrisy in his statement.
The pair decided to kill their shared father to protect their love child, and right before Amy's divorce party, Heath lured Roger to the gazebo, under the pretense of finding a problem with the hanging mannequin, only for Brianna to appear and their plan to commence. Heath chloroformed Roger, allowing Brianna to place the noose around her father's neck, killing him via hanging before leaving the scene. Amy ended up suspected of the murder due to her bitterness, and suspicion turned towards Katherine after Roger/Trent's reveal as a bigamist. However, nail polish remover ended up revealing Brianna as one of her father's killers, as it was found all over her hands.
Brianna and Heath's relationship was revealed during their interrogation, and regarding killing their father, the evil Brianna stated that she and Heath had to protect their baby. Following their confession, Brianna was arrested (off-screen) along with Heath for their father's murder and obviously for incest.