Retain an attorney ASAP and avoid talking to the insurance company until you have had an opportunity to do so.
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If you have a claim against a municpality or government entity, there are time limits that you have to provide notice. For private businesses, you must abide by the statute of limitations. You should discuss these issues with an attorney as soon as possible so you don't miss any deadlines. I'd be happy to discuss your sister's case, and feel free to contact me with any questions. It is much better to have an attorney deal with the insurance company that to have your sister go at it alone.
You should consult with a local attorney ASAP. Do not deal with the insurance company yourself. As for proving your sister's claim, did she get the names and phone numbers of any of the eyewitnesses? If so, I would provide that information to the attorney you consult with so that he/she can contact them in order to collaborate your sister's story with to the restaurant's insurance company.