Sometimes the police charge both people or the wrong person during an altercation. Proving that your sister's involvement was not criminal will require a talented criminal defense attorney. People often feel like they don't need a lawyer when they're innocent because the truth is so obvious to them. Your sister might be the only one charged and the other girl will probably be a witness.
Your question is vague. Was your sister cited for assault? Were you a witness? How did your sister assault the other woman? Why are they fighting? Try to ask one question at a time or you will confuse everyone.
It's a little difficult to follow your question, so you may want to take the time to rephrase it. However, sometimes a cop will cite both people in such a scenario and let the prosecutor's office sort it out later. Your sister's best bet is to hire a local criminal attorney to fight the charges against her. Good luck.