You didn't say where you mom lived at the time of her death. If she lived in Cook County, Illinois (for example) then I would suggest that you check with the Clerk's office in the Probate Division to see if a Will was filed for your mother. If there is a Will, as you believe, then the person holding the original must file it with the Clerk under Illinois law - they can not hide it once a person dies.
However, the bigger question, is whether you know what assets your mother owned at the time of her death. Did she own real estate? Was it with her husband (not your father)? Maybe you need to look at the record's in the recorders office to investigate that matter. Is the other property that she owned valuable or simply important to you?
Good luck in obtaining closure.