I'm sorry about your mother's death. If your mother had a will, California law requires that it be lodged with the probate court within 30 days after death. If your sister is not willing to start the probate, then you can file your own petition to probate the estate. Contact a probate lawyer in the county where your mother resided to find out more information.
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Attorney Brewer is absolutely correct. The executor is required to file your mother's will with the court within 30 days of her passing. If she does not do so, and does not commence probate proceedings, you should consult with an attorney, as you may file your own petition for probate.