http://www.ilga.gov/legislation/ilcs/ilcs4.asp?ActID=2104&ChapterID=60&SeqStart=3700000&SeqEnd=5000000[/url]
After the debts of the estate are dealt with, the proceeds are distributed in equal shares to your parents and siblings. If one of the parents dies, the surviving one gets a double share. If a sibling dies but has descendents, their descendents divide their share.
In your case, 1/3 to the father, 1/3 to the mother, 1/3 to the sibling.