Hi Leah
Can you ask the service she works for if they have a system in place? I know from past posts that a lot of your fee goes to them, so it doesn't seem unreasonable to expect that there would be a system in place to let clients know if she died.
The other thing is, Is she married? As she has your details, and as you say you are her only long term client, could you just ask her to pass on your details to her next of kin, so that they could let you know if she dies or is unable to communicate with you? I would imagine that people might keep some sort of list of people to notify of their death...
I hope you can find a way to organize this with her, so you can stop worrying about not knowing.