|There are quite a few SOHO Router's that come with the ability to limit bandwidth. If you don't own one already, shop around and buy one that does.|
If you already own a router, see if it's compatible with Tomato Firmware (google it and read the compatibility info on Tomato's web site). If it is, then install the Tomato firmware as it allows bandwidth metering.
If you already own a router and it's not compatible with Tomato, or you can't afford a router, I would recommend talking to your cousin and explaining the situation to her and ask her to stop. If you're a gamer and it's interfering with you gaming, you could always unplug her from the router for the duration of your gaming session and plug her back in again when you're done.
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.