My apologies. As you should be well aware, gender is not readily available on this medium, and it is culturally custom for most to use the term "he" when the gender of someone is unknown.

Anyways, as per the terms and conditions provided by National both "earned" and "one-two-free" days are non-transferable. "Earned" free days are impossible to transfer from a technical/system point of view. However, "one-two-free" days are just coupon codes, and there are no controls to prevent someone else from using the code, but it is I'm violation of the terms. In the (highly) unlikely event that the reservation got somehow audited it could be canceled. In practice, no one has reported experiencing this, and you would probably be safe to make a reservation in your sisters name using the free day coupon codes. Just be aware that it is against the rules and you are doing so at your own risk.