Boys, partricularly, but girls sometimes, will establish the pecking order - a normal activity. Wrestling is most common, but the occcasional slap or fist can pop up in the heat of the moment. Generally speaking, no big deal and its usually followed by an apology and true concern for the injury.
Now, if the boys are literally duking it out, with black eyes, bloody noses, and lost teeth, this is not reasonable. Fighting to injure is not acceptable behavior.
Family members need to clearly understand that there are limits when dealing within the family (and ultimately with all people). This self control is sometimes difficult to master in adolescents, but should be well developed in older teens and adults.