Sons being abused by daughter

So my 10 year old daughter and 13 year old son were arguing over their favorite chair and it got pretty heated. She's slapped him and pulled his hair before, but tonight she kicked him in the groin as hard as she could. He was on the floor for quite a while and he's still pretty sore. I'm scared she could make him sterile if she did that again so I grounded her for a month would you say I done the right thing? Xx