My mother battered my sister, in self defense my sister bit her to get away.

First, it was a very good idea for your sister to get a report filed and photographs taken of her injuries. As to whether or not the DA will file charges it is really up to them to decide; you can hope that they won't but you cannot control whether or not they do. It sounds like your sister would have a very good defense of "self-defense"; the law permits a person to use reasonable force to protect themselves, or others, from an imminent assault. Self-defense, however, is what is called an affirmative defense. This means when someone is charged with assault they have to affirmatively put this defense forth before the jury. Ultimately, a jury will decide whether what happened was self-defense or not. I'm not sure but from what you describe it sounds like you may have been present when this happened, if so you certainly would be able to testify for your sister as to what happened. Additionally, even if you were not there you certainly could testify on behalf of your sister as to the long-standing history of physical abuse that you and your sisters suffered. If your sister gets charged the first thing that she is going to need is an attorney. She should seriously consider not making any additional statements to the police until she has spoken to an attorney.