Legal Question in Wills and Trusts in Illinois

The wife's sister most probably has no relationship to the estate of the husband.

Likely, the answer is "no," particularly concerning the things you are probably considering. That said, an attorney working through the estate may find something that needs to be separately addressed. Also, I recommend searching the state's I-Cash database for unclaimed property: https://icash.illinois.gov/index.asp, as well as similar databases in other states to which there may have been a connection.