If subpoenaed to come to court, then she must appear or be subject to penalties (including being arrested and held on a witness bond.) The charged sister obviously needs a lawyer to represent her. I would strongly suggest following whatever strategy the lawyer suggests. My guess is that this case will never get to trial, but one must be ready for trial regardless.
The basic problem that the sisters have is that once a criminal charge is filed, the State has full control. The "victim" may have an opinion, but the State (the prosecutor and the police) are really the one's to decide what happens. The crime is against the state, not the person. The "victim" is merely a witness.
I agree with the prior answer, that if served, the "victim" should appear in Court. The Defendant needs a lawyer.
In an effort to address your question directly, very often, an attorney (for the Defendant) can talk with the prosecutor prior to trial to see of there are ways to resolve the case short of trial. The nature of these discussions will depend on the circumstances of the crime, who the prosecutor is, and a variety of other factors that can be dealt with by a local defense attorney. In short, an experienced local defense attorney is every one's best bet.
The State Attorney's office in Snow HIll (That is where the county seat is) tends to be reasonable. They know that witnesses frequently do not show up and I managed to get reasonable dispositions of a few cases there. I am sure that a local attorney should be able to help you.
I agree with all of the previous answers and just want to share some additional thoughts from when I was a prosecutor and how that law enforcement officer thinks in situations like this. First, the accused sister must get a lawyer and if that lawyer is able to speak to the complaining sister, communicate to the State's Attorney that no charges are being pressed. If the prosecutor insists to prosecute, then he/she will have to find a way to introduce evidence of the assault independent of the victim. That's just the nature of the training you receive as a prosecutor because you never know if the complaining witness will ever show up. That's not advice for the victim not to show up - she should appear in court if properly subpoenaed or face a body attachment (a court order for arrest for violating a subpoena). But the simple reality is that the prosecutor must be ready for that and get either the 911 tape, or other witness statements, which unfortunately may come into evidence under a case called Davis v. Washington from the Supeme Court that was issued in 2005. Often times, as one other member of the bar mentioned, you can broker an agreement with the prosecutor and get charges nolle prossed. If you get that dismissal, don't forget to expunge the charges right away.
Have the sister who is the State's witness contact the prosecutor immediately. She should tell them that she does not wish to go forward, and that she wishes to not appear as a witness for the State. She should, however, be careful what she says. If she admits that a crime took place, the prosecutor could force her to testify. Also, it appears that the police officer is also a witness.
Most likely, the prosecutor will hopefully drop the charges. I do recommend that the other sister obtain an attorney just to be on the safe side.