This is unbelievable! The VERY least that Cox could have done was to assure you that they had noted the account and that once a death certificate was provided they would dismiss the charges.

It sounds as if you did indeed inform Cox that charities were coming to take property from the premises, so they needed to retrieve their equipment by a certain day. I see absolutely no reason they couldn't have required someone be on site as they took the equipment away (which they would have to do anyway, as they would need a means of getting into the house). That to me is as good a proof as any that the home owner is "agreeing" to have their service disconnected.

Sorry about your sister! I'm sure it's an awful situation to go through!