Firstly, very sorry to hear about your Mother-in-Law, it must be a bad shock to the family.
I'm not sure I understand why your insurance company want a letter stating you've cancelled your flights. It must be obvious to them from medical evidence that you can't go. However, what happens if you go to your account online with Easy Jet - does it show there that the flights are cancelled? If it does, take a screenshot, paste it to Paint, save it and copy the image into a letter to the insurance company. It's proof enough I would have thought.