Legal Question in Real Estate Law in Illinois

From what you describe, your mother is a month to month tenant. This means that the sister in law can give proper thirty day notice, and if your mother does not leave the property by the end of the next rental period, the sister in law could file an eviction action with the court. Simply put, your mother does not need to leave until a judge tells her so. However, there is nothing in what you describe that would indicate to me that your mother is entitled to live in the property for life. So if sister in law wants her out, her eviction likely is an inevitability. It would be a good idea for you to have her talk to a landlord tenant attorney.