There has to be more to the story than this. But don't publish any more details here: hire a lawyer or use a public defender.
PC 13700 is not really a separate charge. It simply defines some of the terms used in domestic violence cases. PC 273.5 is a very serious charge for which she will need a lawyer either public or private.
Getting an attorney to begin the investigation right away is important. Pictures of your sisters injuries and interviews of the people at the house are important to try to determine what occurred. Don't let her talk to anyone until she talks to an attorney.
Response to answers is for general information purposes only and does NOT establish an attorney client relationship. Further, without a full client interview, opinions are based on limited information and therefore a approximation. Actual advise may vary significantly once addition information is established.