Edit : I see the model# in the title. That Dell did not come with a wireless network card built in. You will need to purchase one and install it. Luckily your dell has usb2.0, a simple usb wireless device should work fine just make sure it has drivers available for it that match your operating system.
If your sisters "kit" included a wireless router then you may have the option of running a network cable from the router to the Dell. Not all wireless routers have wired ports on them. Check the back of hers. You usually can pick up a used network cable from a computer outlet/clearance for a few bucks if you dont have friends with free ones for you.