How do I claim assets that are in probate in Illinois if my relative had no will and I am the only living heir?

Elderly Aunt who has been estranged for decades. She has no husband or children and her only sibling, my father, is deceased. My brother and I are her only living relatives and we both live in different states. I have no idea how to proceed and I do not know if I can trust these companies who are now starting to contact me offering help for a fee if I recover any assets.