It is not wise to just plead guilty to this kind of offense. Even though a Class C is a fine-only offense, it is an assault conviction that will stay on your record for the rest of your life.
However, you should also be aware that even if your sister does not show up at court, the State still may be able to prove that you committed this offense through other types of evidence. Specifically, they may be able to admit the 911 tape as well as any statements that you or your sister made to the police when they arrived at the scene, as well as anything the officers personally observed. A lawyer might be able to keep some of these things out of evidence which could very well prevent you from being convicted.
(The other option is to get the prosecutor to give you what is called a deferred disposition. Basically, you plead no contest and as long you do some community service and stay out trouble, the case is dismissed after 3-6 months. Some courts don't offer them on this kind of Class C, but it won't hurt to ask.)
I would recommend at least speaking to a local criminal defense attorney to see what advice they would give you and what kind of fee they would charge you on this case. You are not entitled to a court appointed attorney because this charge carries no jail time, but you might wan to make special arrangements to hire a lawyer to protect yourself.
Disclaimer: This answer is provided as a public service and as a general response to a general question, it is not meant, and should not be relied upon as specific legal advice, nor does it create an attorney-client relationship.