I am sorry for your loss. Apple will accept the death certificate, however, you will need to file a request online. The in-person Apple Stores don't accept requests to access a deceased person's Apple Account. Secondly, in addition to the death certificate, you will need legal documentation that states you are the rightful inheritor of your sister's estate.
Once you have access to the Apple Account, you can remove Find My off of her iPad, unlocking the device and making it usable to you.
For steps and to file the request: Request access to a deceased family member’s Apple Account - Apple Support