There are many ways to this problem. You can
1st) Install Ethernet work cards in both ur sis PC and yours
network using either through a hub or a special cable that link two
ethernet cards together (pardon me, I forget the cable name, I have
to chekc it out). The latter is a much more cheaper choice than the
hub. Install a internet sharing program and you and ur sis can
surf!!! Bad point:- Your PC have to be on if your sis want to
surf.
2nd) Buy another ADSL modem. Bad point:- Cost $$$
3rd) Depending on the Modem type,some modem provide ethernet
connection, so unlike to option 1, both pc using ethernet cards can
route directly into the modem via a hub. Bad point:- Despite using
a hub (which cost $$), you have to deal with the software
installation!!
Just my 2 cents worth!!