question about connecting to Internet

[Lirange]

So on impulse I bought a new computer online and now I need to figure out how to connect that and my now sisters computer to the Internet. We have cable that is connected to a router. I'm going to be gaming so I need the best connection. Can I just hook up my desktop to my phone line? Or does it have to be connected to the router? If so I can move the stuff to my room then ill need to get my sister Internet. I know there's a usb dongle you can buy that let's a desktop connect to wifi, does anybody know how reliable those are? The most bandwidth she'd be using would come from streaming youtube videos.