Specially since they are not mutually exclusive, your sister probably felt better after you told her "i kicked his ass and told him to stay away", than if you just said "forget about it, in five years this won't matter".If someone bully your little sister...would you go to console her and don't pay attention to the bully? or would you threaten him (or even fight him)
Just common sense. I was in some fights when i was a kid defending my sister, and believe me, sometimes works