Legal Question in Wills and Trusts in Illinois

First place to look is the documents if there are any. Is one of the siblings appointed executor or trustee? If so, then they should account for what they have done and distribute the proceeds collected in accordance with Illinois law and the terms of the document. If there is no will or trust document, and the value of the assets warrants taking legal action, you can open a probate proceeding as a way to recover any assets wrongfully liquidated by the other sibling. Best first action, talk to the other sibling and ask what's going on.