Lying to the police is never a good idea.  Depending on what you are lying about is how you could be charged.  If you lie during an investigation then you could be charged with obstruction.  If you lie about a crime al together  - that it really never happened - that is a charge in an of itself.  Here you are already charged - or your sister is - with OWI.  You must know that you have no created a HUGE mess for your sister and you need to consult with an attorney right away about putting things right.  The arrest record is going to show up under her name even though there will be no conviction once the matter is cleared up.  She may need to expunge the charge. Speak with an attorney please.  DO NOT go to the next court appearance without one and really, fess up before.  It is the best thing for both of you.  Good luck.