It sounds as though you first need to find out if your mother is still alive. Seeing how she is probably mentally incompetent, she would be unable to alter any estate plans at this point to include you. Very often, the child closest to the parent in the parent's old age will have the parent execute a power of attorney to conduct their business affairs.
This case requires a lot of due diligence to locate your father's will, your mother's whereabouts, and any attorneys or others involved in each of their estate plans.
Because of the potential size of the estate with you as a beneficiary, it might behoove you to hire an attorney to begin the investigation.
If you need further assistance, you may contact me via email.
Good luck!