Why bother donrael? Instead, why don't you open a new checking account there, (maintaining the FDIC coverage you seem to like), send the checks here to your sister, and then when she needs $$$, make the deposit, and then call her to write a check in that amount and change it at a casa de cambio.
Just tell her to not bounce a check at the casa de cambio, or she will be on the hook for 2% A DAY in interest on the amount of the bounced check, and out of business for getting her cash.
Immediate cash for her, no cost to either of you (except maybe a small monthly service charge for the new account).