I see why she does it a lot.

Sarah like to tease her step brother Danny who she knew had a massive crush on her by squealing and moaning loudly whilst 
pleasuring herself as she knew he could hear her through the wall and it drove him mad that he could not join in.

Danny had had enough of this and decided it was about time he did something to teach her a lesson.

Feeling no guilt or remorse for doing this to her he waited until he heard her playing with herself before swallowing the potion that allowed him to take over her body and have his wicked way with her without her knowing he had.