Being Sued By Heir For Title Of My House

Illinois- I was informed by my title company that I was being sued by a stranger for the possession of my home. I bought it 3 years ago from a flipper who bought it from a widow. I guess the widow was never on the title and there was a prenup that said she wouldn't get ownership after the husband passed. From my understanding, there was never probate. The heir has now opened probate in order to take my house. None of this was known to me when I bought and it cleared title examination with a reputable title company. So my questions are 1) how the heck did it clear title examination and 2) what's the worst case scenario and most likley scenario? I love my house and don't want to lose it!
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