Right off the bat it does look like a one-off, so
it may be difficult to determine a value because
there are no comparable sales. Have you tried
running it on compressed air? It will be worth
more if it is a runner.
Ultimately, the only way to find its value is
to put it up for auction. Maybe a 'Buy it now
or best offer' type listing with a high starting
price to make your sister happy.
Good Luck,
Wayne