This is not really a legal question but it is clear you want to be thoughtful. I don't know that there is a painless way for you to accomplish what you are attempting, but you might start by sending her a card or letter and making contact that way. In that card or letter ask her to contact you as you want to discuss a small matter with her. Hopefully she will contact you and you can discuss it with her. Otherwise you just have to move forward and ask for what you want/need. Good luck.
What are you trying to achieve and why? The only way I know that you would be able to get this is an order of the Probate court acknowledging you as the surviving spouse if your mother in law wont cooperate. See http://www.cdph.ca.gov/certlic/birthdeathmar/Documents/Affidavit-DeathPAMPHLET-(01-13)-MERGED.pdf
You would have to file for probate.
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