Sorry to hear of the death of your father and resistance from sister. Often people believe trusts are great to avoid probate, but with trusts there is no court protection without docketing the trust.
In this case, it sounds like sister may have some self-served interest. But, without seeing what the trust allows, it would be difficult to truly advise you of the correct procedure. I cannot believe the attorney for the trust does not have a copy. If he or she does not have a "signed" copy ask him to mail you an unsigned copy. If she doesn't have stamps, mail her a self-addressed stamped envelope. Or if the attorney is assisting your sister, ask the atty to ask for a copy, then he can forward it to you. There may be inheritance tax issues that will require appraisals, and such.
Does the other child have a copy or be willing to side with you to request it? Again, without seeing the trust, if the trust provides equal distribution to the 3, then yes she would have to purchase the other shares or the house/business would need to be sold.
Did the Trustee have a POA of your father? I would hope that assets were not moved from the trust. You can verify the ownership of the real estate by calling the county assessor.
I would send one last letter demanding a copy of the trust and a list of the assets owned by the trust by a certain date to avoid your need to hire an attorney. What County is this in?
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Arlene Kline