The laptop is 802.11g compliant which is one of the modes supported by the router.
OK - non techy as best I can.
Open up a command window - START - RUN - type in CMD , click OK.
In the black box that opens up type IPCONFIG.
Note down the numbers that are the IP and Default Gateway. Likely to start 192.168.x.y
Now open up a browser session and in the address bar type in the numbers that were the default gateway and press enter.
You will likely come to a login page - I can't help anymore with this one
Find the setup page/s and specifically look for the wireless setup and report the details back - or get your sister to do a screen dump. If needs be attach that screen dump to this thread.
Then we can talk some more ...