Sugar and spice and a tongue like a knife

We have a "force" in our family. They are our dear daughters..one 14 and one 15. They're the best of friends and rarely do anything without the other close by. They can be thoughtful and sweet, lots of fun unless they gang up on someone. And lately their 17 year old brother Steven has been their hapless victim. If he utters a single word, both of them roll their eyes and look at him as if he just came off the bottom of someones' shoe. They obviously find him to be beneath themselves in every area. He looks, acts, speaks and even chews wrong. It's been getting out of hand. I've spoken to them about how unkind they're being, we've looked up verses about being loving and humble; there's been some small attempts to correct their attitudes but mostly they're just nasty to him. Tonight at dinner one of them answered Steven with a look of pure disgust. Their father at that very moment decided that he needed to step in and help with this problem. The offending daughter was sent to her room for the night, no movie with the family, no Youth Group. I won't say which girl it was but I will say that Grace should be done with her 9 hours of dialysis much earlier than usual tomorrow morning.