Almost certainly your sister's new machine will be preloaded with a trial version of either Norton or McAfee anti-virus software.
It's obviously up to her whether she decides to continue with (and pay for)one of these after the trial period - both are excellent products - or whether she decides to opt for the free version of another product. Other forum users may mention the free version of AVG, which is very popular, and rightly so. It provides basic protection against computer viruses without all the bells and whistles associated with its (and others') paid for versions.
In the end it's a matter of personal choice and depth of pocket - free software like AVG will keep you safe from viruses, but that's all.