Simple assault is the more serious offense.
Section 2701 of the Pennsylvania Crimes Code defines simple assault as either: (1) attempting to cause bodily injury to another person, (2) intentionally, knowingly or recklessly causing bodily injury to another, (3) negligently causing bodily injury with deadly weapon, (4) attempting by physical menace to put another in fear of imminent bodily injury, or (5) concealing a hypodermic needle and intentionally or knowingly penetrating a law enforcement official with that needle.
If the defendant is an adult and the alleged victim is under 12 years old, simple assault is a misdemeanor of the first degree. Participating in a mutual fight constitutes simple assault of the second degree. Otherwise, simple assault is a misdemeanor of the second degree.
Section 2709 of the Pennsylvania Crimes Code defines harassment as the intentional harassment, annoyance or alarming of another person through striking, shoving or other offensive physical conduct or through following the other person in public places or by using lewd or otherwise offensive words or conduct. The offense is graded as either a summary offense or misdemeanor of the third degree depending on the nature of the conduct.
Simple assault and harassment are often charged together. Without more specifics on how your sister's injuries were caused, it is difficult to say which is the more appropriate charge in her case. The police may have determined, for example, that there was no intent to actually injure your sister. Nonetheless, it is the prosecutor's office who will eventually determine the charges.