My sister has taken care of my father for may years. She is on his bank account. I know there is a will but she will not produce it .

You do not indicate if your father is alive or deceased.  Unless he is deceased, there is no requirement that your sister produce the will.  Upon his death, the Probate Statute (755 ILCS 5/6-1) states any person who has the testator's will in their possession has a duty to file it with the clerk of the court of the proper county (meaning the county your father resided in or held real estate in).  The statute further states, if anyone willfully secretes, alters or destroys the will for the period of 30 days after the death of the testator is known to him/her, the person if convicted of such will be sentenced as a Class 3 felony.  If your sister has secreted the will, you should file a report with your county sheriff.  Hopes this helps.