There would need to be a router if your sister wants her PC connected at the same time.
If its a wired connection and a router then nothing, just an ethernet cable, CAT5 or better. Plug one end in to your laptop and the other in to the router.
If its wireless and wireless is on the laptop then just right click the wireless icon beside the clock and select connect to a network.
Enter the SSID (the name of the wireless access point eg linksys) and select the security eg WPA2 and enter the passphrase then hit connect.