Legal Question in Real Estate Law in Illinois

It sounds like the term "own" is being used in a practical sense, not a legal sense. You need to get things straightened out in a meeting with a qualified attorney who can review whether to change the ownership or recommend wills or other estate planning documents.

As it stands, your sister would not have any clear interest should something happen to you. Similarly, her heirs or legatees would not have any clear interest should something happen to her. Also, nothing would happen on her remarriage as far as title to the property, but your sister's ownership interest may come into play between her and her husband should there later be a divorce.

If changing the title can be done without any tax consequences, then it should be considered, as should an adequate estate plan for each of you. This may come down to paying for services now or everyone paying a lot more when the unavoidable issues come around.