is it a wireless connection?
ur sis's com might have been configured to auto logon to someone
else's new wireless internet. can go to the internet connections to
double check its loggin to the correct network.
or, incorrect settings for dhcp. nt enough spare lan ip to go ard
for the dhcp to assign. set to dynamic. max no of ip address. can
even try lengthen dhcp refresh time.
or, bad proxy server. routing through a bad server might cause the
internet to nt connect to anything properly. turn off the
proxies.
if really wireless, try setting transmit signal to max.
move close to the modem,
or lower RTS threshold level,
or lower beacon interval to achieve better synchronization.
or, better still, jus use a lan cable to connect directly to the
modem and see if it works so can isolate problem.
unlikely to be port problem. ports allow outside programs or
inside programs to communicate. wont cause internet to nt work.