That is unfortunate, but you will not be able to use that phone until the account it was tied to is completely paid. There is no way around it, sorry.
You might be able to get a phone inexpensively on ebay, but make sure you get the
ESN from the seller and call Sprint to make sure the phone you're buying is clear for activation or you might have the same problem again.
Also, does your sister know if she doesn't pay Sprint it's going to tarnish her credit and she won't be able to get loans or credit cards and if she ever signs up for a new contract with ANY service provider they will see the unpaid Sprint bill and charge a very large security deposit.