Q:

Why does Jeanie help Ferris out at the end of Ferris Bueller's Day Off?

One thing I have never understood about one of my favorite movies, "Ferris Bueller's Day Off": why does Jeanie decide to help Ferris out at the end of the movie and save him from being busted by Mr. Rooney?
She has spent the entire movie expressing her hatred for Ferris and finally has a chance to show her parents that he skipped school, yet she lets him off the hook and saves him from both her parents and Mr. Rooney.
At IMDB, someone has offered the explanation that it's because she doesn't want her brother to keep going to school with her, which he would be if Rooney holds him back and doesn't let him graduate. But I find this answer unsatisfying.  
Also, it's been suggested that perhaps her therapy/makeout session at the police station with Charlie Sheen helped change her attitude.  But I find this answer unsatisfactory too, because if this was the case, why did she speed home from the police station, hoping to get there before Ferris?
Can anyone offer up a better explanation for Jeanie's change of heart?

A:

I never really considered it to be a territorial issue as much as it was for her to seize the only win-win outcome for herself:
Jeanie already faced-off (or rather, kicked in the face) Mr. Rooney, mistaking him for a normal burglar.  Seeing that he was correctly pursuing Ferris for truancy (meaning Ferris was truant, not that his methods were justified)--and knowing the futility of law enforcement in the situation--she claimed her win over Mr. Rooney here by excusing Ferris' behavior and absence, which Mr. Rooney could not contest.
With Ferris, it is true she failed to justify hers and Ferris' inequitable treatment, as we remember from her voice-over in the school hallways ('why should Ferris get away with everything? ...after all I got a car, he got a computer...').  But, I speculate that getting home before Ferris would be her way to gain leverage over him.
The opportunity, however, to humiliate/deprive Mr. Rooney and indebt Ferris ultimately was probably more appealing than the inevitable shattered, broken happy home that would occur when their parents' found out he isn't such an angel as they believed.

A:

When you have a little/big brother or sister, you hate them...you would do anything to get over on them or even hurt them sometimes...but when someone else tries...they become YOUR territory that is being threatened...you should defend your territory. This was Jeanie's knee-jerk reaction.

A:

I thought the motivation to switch to Ferris' side was pretty clear.  Her mind was changed while she was in the police station chatting with the cool and laid back character played by Charlie Sheen (see it here). He professes a philosophy of live and let live and tells her that she is the problem—not Ferris.  She rejects the advice several times, but eventually embraces it when she falls for Sheen's character.

