You could use two routers. Depending on the modem, it may even allow two networks joining it, but more likely you will have to have one router be the head-in, while the other is subordinate to it. This strikes me as the most likely scenario to work, but there are questions that come to mind immediately. For example, how does your sister connect? If it is a Wi-Fi connection, it will be difficult to completely harden both routers to the point where only certain machines can get on one, and not the other. You could make MAC address restrictions, I suppose, but you will need to inventory every IP device in the house - iPods, televison gear, routers, PCs, etc. Another question - will she need to print to a printer on the non-sister network? If you go this route, here is Cisco's way
of doing this with retail Linksys routers.
There are switches which act as routers by making VLANs, but to do that AND harden them correctly would be beyond the scope of someone "new to this."