Wheez
I am in the final round of a four-year fight with my half-sister to have over $1,000,000.00 returned to my father's estate that she misappropriated while acting as agent under a POA. I have learned ALOT during this time.
I assume that you and/or your mother are "interested parties" in your father's will? If so, (and if the Court still has records) you need to obtain a few CERTIFIED COPIES of your father's will and a COPY of the "Inventory of Assets" that your brother filed with the Probate Court when your father died. From there, and as an "interested party," you will be able to request your father's tax information from the IRS under the Freedom of Information Act (The irs.gov website has instructions on how to request information under the FOIA). You will also need to obtain a few "Certified" copies of your father's death certificate and send the IRS one of them along with your request. You will also need to draft a "Letter of Intent/Instruction" outlining the details of your request and have it notarized.
In your "Letter of Intent," you will want to request all "Attachments" to your father's returns for the years specified. Specifically, you want to request all 1099s (MISC), all 1099s (INT), all 1099s (DIV), W-2s, etc. I suggest that you request this information for one year prior to your father's death, the year of your father's death, and the year following your father's death. These "attachments" will provide you with a wealth of information such as name & address of holding company, dates and amounts of assets sold, etc. From there, you can contact his brokerage accounts directly. (Not all asset companys will give you the information that you are seeking, but some will. You may have to hire an attorney to subpoena this information if you can't get it on your own.) You can also request your father's tax returns for these years. Make sure to also request the attachments as above along with the returns.
I was able to locate (7) of my father's brokerage accounts and over $1,000,000.00 in stolen estate assets using this method. Give it a shot.