|Considering what you said above, there's only one thing I can think of doing. Cut sister off.|
From the sounds of it, she'd doing something very bandwidth intensive. Which to me says either video streaming or, downloading/uploading of illegal movies, software, and/or music.
Here's what I suggest............the next time sister is connected and hogging all the bandwidth, go pay her an unannounced visit and have a look at what she's doing. If she's a reasonable person, perhaps you can make an arrangement where she doesn't hog all the bandwidth all the time. If she's unreasonable, and doing something illegal, maybe you can blackmail her by threating to rat her out to your parents.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.