How can a 273.5 (A)PC Inflict Corporal Inj be dismissed with 13 eye witnesses.

The District Attorney either dropped the charges or decided not to charge him because there was insufficient evidence to prove beyond a reasonable doubt that he was guilty of a crime. This can happen for a number of reasons. Sometimes there is evidence of "mutual combat" or self defense which would make a conviction difficult. Whatever the reason - your sister can contact the victim witness division at the D.A.'s office and inquire and give more information if she is inclined. Your sister should contact WEAVE here is Sacramento, if she has not already, for advice and counseling about her situation. They can assist her in obtaining a restraining order to protect herself and any other persons (does she have children?) If she can afford an attorney that's great - but she can proceed with a restraining order at the Family Law Court even without an attorney and she probably should if her husband is capable of that kind of violence. If she has children, she must seek a restraining order and protect her children or risk CPS involvement.