I don't understand what you're trying to do, but you can't just put a phone line into your computer, you have to go through a modem/router.
So can you get the router in your room so you have direct cable connection to your computer?
I've got one of these "
dongle" thing for my laptop (coz the in-build Wifi was next to useless). And it worked fine enough for me.
As long as your sister doesn't complain then it's fine =D