You are a creditor of your mother's estate, not your stepfather's. The community estate of your mother and stepfather would be liable for the funeral expenses. If an estate has, or will be opened, you will have four months from the date of the appointment of the administrator/executor in which to file a claim. It might be worth looking at the terms of the 401k to see what is provided upon the death of the principal and the subsequent death of the beneficiary. Some times there are specific provisions as to who succeeds to the fund..
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I'm sorry for your loss. While each state has its own specific rules, every state has a process whereby creditors of the estate can make a claim against the estate and its assets. You are a creditor and should contact your stepsister, but also take the precaution of contacting the probate court in the county where they lived and ask about how to file a claim. If you cannot get the information you need, you may need to consult with a California lawyer to find out how to file the claim. Good luck.
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My comments are not intended to establish an attorney-client relationship, are not confidential, and are not intended to constitute legal advice. Proper legal advice can only be given by an attorney who agrees to represent you, who reviews the facts of your specific case, who does not have a conflict of interest preventing the representation, and who is licensed as an attorney in the state where the law applies.