Here are the rules from National:
Renter must present an acceptable credit card, with available credit, in their own name, at the time of rental.
Signature on back must match name imprinted on card.
Credit Cards honored:
* American Express
Carte Blanche
Diners Club
Discover Card
* MasterCard
* Visa
JCB International
While National accepts all of the above cards for rental payment, some of them are not yet available in our Emerald Club features. The "*" indicates which Credit Cards are available for use in our Emerald Club features.
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Debit and check cards may only be used as a form of payment at the end of the rental (once the vehicle has been returned). National Car Rental will not accept debit cards at the start of a rental. A credit card or cash qualification is required.
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All drivers must meet the renting location's minimum age requirements, have a valid driver's license, clean driving record and present a major credit card in his/her own name at the time of rental. See also "AGE REQUIREMENT", "DRIVER'S LICENSE", "DRIVING RECORD" and CREDIT CARDS".
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I would call National and talk to them about this, don't wait until you get there. Is your sister under 25? If she is, that may present another problem.
http://www.nationalcar.com/