Back in January, the two became entangled in a heated argument while in the home of their parents in Lund in southern Sweden.
In court, the girl admitted that she had hurled a number of verbal insults at her older brother before unleashing a flurry of kicks and shoves, the local Skånska Dagbladet newspaper reported.
According to the boy, his little sister started the fight by making fun of his girlfriend.
Having sustained a number of kicks from the little girl, the boy responded by punching his sister twice in the face before their mother intervened to put a stop to the altercation.
In court, the boy defended his violent response, claiming he was acting in self-defence.
But the district court rejected the 15-year-old’s self-defence claims, citing the boy’s own admission that the kicks from his little sister didn’t hurt that much.
While an assault conviction normally carries a prison sentence, on account of the boy’s age and the fact that he hadn’t had any previous run-ins with the law, the court opted for a more lenient sentence of a fine.
According to the paper, the siblings explained to the court that they hadn’t spoken to one another for a week following the fight.
However, time had since mended the rift and they had once again become friends, they told the court.