It's not just bidders that stuff like this
happens to.  I haven't sold anything on ebay,
but I have bought a few things.  A few years
back before I went to DVD's, I won an auction for
a used copy of a Ranma 1/2 VHS.  I emailed the
seller three times within a week and a half of
the auction ending and got no response at all.
Finally, I gave up and posted negative feedback 
stating that.  Within like two hours, he had
retaliated with negative feedback stating that he
had sent me seven emails without a reply, which
was an out-and-out lie because he also emailed me
stating that his email had been up and down lately
which was why he hadn't been able to receive my
emails or email me and bitching at me for leaving
negative feedback.  The kicker was that the
account that he was emailing me from was different
than the account that he had registered with ebay,
which was probably the real reason.  (Oh, and he
was also one of those annoying people who DO NOT
KNOW HOW TO TURN THE CAPS LOCK OFF).

In your case, I would give the buyer two options:
- Repay for shipment
- A refund minus the cost of shipping

Though, if your sister doesn't want to go either
of those routes, send it media mail (4th class
mail...used to be book rate)..will probably be
only $1-2.