Mr. Perry's answer really could not be better.
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Your stepmother could not change your father's will. The terms of the trust will dictate whether she has the power to change it.
If you are a beneficiary under the trust, you are entitled to a copy of the trust document. You may need to hire a trusts and estates attorney to write to your stepmother and ask for a copy, and to make sure that you get any share of the estate to which you and your siblings are entitled.
You should not delay acting on this.
Mr. Perry is correct. One additional point. If it has been more than 60 days since your father's passing, your step mother is in violation of Probate Code Section 16061.7 which REQUIRES her to provide copies to beneficiaries within 60 days of the likely irrevocability of your father's trust (usually irrevocable at death). If she is being secretive now, that is not likely going to change unless you force her hand. A letter from an attorney reminding her of her obligations usually does the trick.
I am sorry for your loss. You have remedies but to exercise them you need to consult and retain an attorney to review the Will and declaration of trust to advise you. If you have not seen the documents, I wonder how you know that one piece of income property was to go to your father's adult children. The trust would become irrevocable upon your father's death. Since the trustee has an attorney it should not be difficult to "get to the bottom of this" in fairly short order once you retain counsel.
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You have a right to a copy of the Trust. If the property was left in trust for the adult child, then its most likely in an irrevocable trust following your dad's death. You can also demand an account of the irrevocable trust either under PC Section 16061 (information not an account but almost the equivalent) or PC Section 16062 depending on the rights in the trust.
You should seek out Trust Litigation counsel immediately.
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The documents need to be reviewed by an experienced Estate Planning Attorney. There are a plethora of issues that could affect the validity of the document and its administration.
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