When Jeannie catches what she fears is a rapist or burglar in her house (actually Mr. Rooney), her immediate response is to kick him in the mouth not one, not two, but three times before running up the stairs and calling the police. And then she uses the intercom to talk to Rooney from her bedroom.
Jeannie, over the intercom: Excuse me. If whoever's in the house is still in the house, I'd like you to know that I have just called the police. If you have any brains whatsoever, you'll get your ass out of my house real quick. I'd also like to add that I have my father's gun. And a scorching case of herpes.
When Rooney catches Ferris, only for Jeannie to bail him out. Not only did she thwart the film's main antagonist in a way that the hero couldn't, but she managed to make Ferris sweat by leaving him at Rooney's mercy for a good long while before deciding to save him. For possibly the first time in her life, Jeannie had complete control over the situation.
Then there's when Rooney glares at her, only for Jeannie to hold up his wallet which "you left in the kitchen." A music sting plays as we get a shot of Rooney's eyes widening, then his mouth opening, realizing he can't bust Ferris without revealing he broke into a student's home.
Then Jeannie throws the wallet off so it wakes up the dog who chases Rooney off the yard.