First off you most likely wont have to port forward because its already on the same network, unless you have some weird setup. If you've downloaded the Minecraft Server program and run it, all your sister needs to do is type "localhost" as the IP and it should work. If it doesn't you'll probably have to go to your router and find your local IP to type in. If you need a more specific tutorial, Google will be your best friend (Although Google should already be your best friend). And if this doesn't work use Hamachi, it is the easier way to go about doing this.