This answer is not intended to provide you with specific legal advice regarding your situation, or to create any attorney-client relationship.
I am going to assume that your stepmother is a Georgia resident for now. If not, the state where she has her primary residence will control and the answer may be different, although I believe that in this particular situation the answer may be the same in most cases. In Georgia, a stepchild and stepparent are effectively legaly strangers to each other for inheritance purposes, unless the stepparent has legally adopted the stepchild (in which case, they are legally parent and child, and no longer steps). In other words, your stepmother's legal heirs, assuming she is not survived by any spouse or biological or adopted children or other descendants (i.e., grandchildren or great-grandchildren by biological or adopted children), will be determined by Georgia law. The heirs of someone with no spouse or descendants, in Georgia, will be that person's parent(s), if either or both parent is living. If neither parent is living, the person's siblings (brothers and sisters) are the heirs, with a deceased sibling being represented by his or her own descendants, if any. If there are no brothers, sisters, or descendants of any brother or sisters, then the person's grandparent(s), if living, become the heirs, and if no grandparent is living it moves to aunts and uncles, with cousins who are descended from a deceased aunt or uncle stepping in line. Of course, your stepmother could have done a Will (or could do one, if she is still living and competent), or a revocable trust, and provided for the benefit of her stepchildren (or anyone else she wanted) at her death. Heirs are only the people who potentially have the right to inherit, and Georgia allows you to disinherit your heirs through properly drafted estate planning documents. Your stepmother could also have named her stepchildren as beneficiaries on assets such as life insurance or IRAs, or as the payable on death or transfer on death designees for bank or brokerage accounts. If she did not specifically take steps to benefit the stepchildren, however, and if she had not ever legally adopted the stepchildren, then the stepchildren have no legal interest or right to inherit from their stepmother's estate, no matter what their relationship was. That is one of the reasons that I recommend people consult an experienced estate planning attorney while they are alive and able to deal with their own affairs.
If you have other questions, you should consider consulting an experienced estate planning and probate attorney yourself, to help you figure out your exact current situation and what, if anything, can be done if your stepmother wants you to receive assets at her death (assuming she is still living). A lawyer you hire can give you specific legal advice based on the specific facts of your situation, which is not really possible in this type of forum.
The heirs of a decedent are those individuals who will take the decedent's property in the event that property is not disposed of by will. The Georgia laws of intestacy set forth the rules for determining the heirs. The word "heir" is used in Georgia law to denote an individual who is entitled through intestacy to real or personal property or both. As a general rule under Georgia law an heir is defined to mean someone of blood relationship (i.e. a child, sibling of the decedent, or more remote heirs) or a lawful relationship (i.e. spouse or adopted child of the decedent). A decree of adoption, whether issued in Georgia or in another state, serves to establish full inheritance rights between the adopted child and the adoptive parents (and relatives of the adoptive parents). Note that stepchildren are not "children" and thus not heirs for purposes of intestacy. It does not matter that a step-parent collected a social security check for minor children or declared them as a dependent. There is a legal technique that might apply but you must consult an attorney regarding a “virtual adoption.” A virtual adoption is not adoption. It is merely a court-given name to a status arising from and created by a contract where one takes and agrees to legally adopt the child of another, but fails to do so. You may have an argument that your step mother agreed with your father to adopt you but never formally doing so.