I agree with all of the suggestions about doing the fully-refundable ticket option. However, I would modify one aspect of it: I recommend waiting to cancel the ticket until your visit is over and your sister leaves the secured area.

The reason is that once the ticket is canceled, the boarding pass granting access to the secured area becomes invalid. From that point, she would be in the secured area under false pretenses, and if an airline agent or TSA were to attempt to verify the validity of her boarding pass in the CRS, she could find herself in some trouble.

Now, it is unlikely that she would have to verify the boarding pass's validity again after passing through the checkpoint, but it's not out the realm of possibility.

I don't think your sister wants to risk having SSSS flags on her profile for a lengthy period of time because she made a mistake on one day.

So, in short, I would wait until she leaves the secured area (and no longer needs the boarding pass) to cancel the reservation and request the refund. And here's another suggestion: for her fully-refundable ticket, she should choose a flight that departs late in the day. That way, she has plenty of time to enjoy the secured area and then leave and cancel the reservation well before the flight would deaprt.

Just my $0.02.