Because the redhead brother was utterly thrashed twice, his body knows any resistance is futile. Since it was implied he never touched the sister, he just uploaded the scandal and got them expelled, she never experienced how imposing Ijin is (like her mother, she thinks she can hide behind her dad's power)
Ijin has a code of honor. Because of this, he earns the respect of a number of former adversaries. If he did bi*** slap her, not only does this go against his code to avoid attacking non-hostile combatants that don't pose a threat, he might lose respect since he already has others there who said they would handle it.