You can't file an annulment on behalf of a deceased person.

I think you are getting distracted from your main point, the distribution of her estate. Was there a will? If so, then no problem. If not, you need to find an estate attorney in your state because the surviving spouse usually inherits all of the deceased assets, no matter how long the marriage was.

You will get no where by claiming that your mother didn't really want to get married.