If the contract provides that your sister was paying only for the photographer's time, and not for any pictures - whether in file or print form - then that's what your sister purchased. From what you have told us, whatever the prior discussion, both the email outlining the terms that your sister accepted and the contract that your sister signed provide that she would have to pay an additional amount for any pictures or disks.
I was not a party to the conversation, and I have seen neither the email nor the contract, so I'm limited in what I can say. Sometimes it's possible to fall back on consumer laws, which may support a claim based upon material differences between oral promises and the written contract, but that's less likely to succeed when the deal is accepted after the terms are stated clearly in an email and in a contract that is apparently so simple and clear that even a first grader could understand it.

