Yes, there is a service fee of $12.50 for Illuminations.
Jenn...I think that if your sister had Le Cellier used for a "reception", then the guests would have not paid to get in. If everyone just met there to have dinner in place of the recption that would explain it. The Illuminations Desert Party is considered a "Hosted Party" and the guests are not required to pay admission.
I believe that is how it goes, that is how our EM explained it to us. I could be wrong or it could have changed in the last few months.