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So sorry to hear of your loss of your father. Adopted children are elgible to take from their adopted parents. Did your father have a will? Does the will disinherit you specifically, or are you omitted (not mentioned) in the will? If your father did not leave a will, then you have standing to ask the probate court to adjudicate his intestate estate, which will be devised according to Illinois law.
If a decedent does not write a will, the state will write one for them. The devise will be to the surviving children in equal shares. Here, that means 1/3 to each of three children.
You can research the history of Illinois statutes to determine what they required in 1990, but you have answered my question. Your mother didn't leave a will, and neither you or your siblings did anything to determine your rights under the intestacy statutes. So by default, your father took everything, not by judicial determination.
Your father's will specifically mentioned you, left you $500, and nothing else. You don't mention whether the will contained an in terrorem clause, which would provide that in the event you were to start a will contest, you would get nothing.
Whatever the intestacy law of Illinois was in 1990 is not relevant to a will probated in 2008. I know this is not the answer you are looking for, but nonetheless it is the answer. Sorry if I can't 'spain it any better, and I wish you all the best with this. XXXXX XXXXX