If there is a probate estate, you can find it at the county probate court. Your father's Will might also need to be probated. You can certainly ask your siblings about this. While you would not be considered an "heir" of your stepmother, you could certainly inherit from her under a Will or Trust. The same is true of your father's documents. Given the timing of events, it is a very good idea for you to consult with a probate attorney to help you figure out where you stand.
I am very sorry for your dual losses. What a horrible time for your whole family!
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Sorry for your losses. Whether or not you are named as an heir in your stepmother's will, you will want to be certain of your rights as to your father's estate. If he had a will, there may be a provision that his heirs outlive him by a certain time period, or if he did not have a will (intestate), you may be entitled to a share of his estate depending on the state he resided in.
A Will is not filed as a public record in Texas until it is probated. A Will must be probated within 4 years of the date of the person's death. So, unless someone has filed an application to probate your stepmother's Will, the only way you can find out about it is to ask her children. Someone needs to probate your father's estate as well. Even though those are two separate probate proceedings, Dallas County will permit you to have them both pending in the same probate court. The heirs of each parent need to hire an experienced probate lawyer but they don't have to hire the same lawyer. In fact, there might be a conflict of interest that prevents one lawyer from handling both estates.
DISCLAIMER: This is not specific legal advice and does not establish an attorney/client relationship.