My only sister's husband turns her to a punching bag.

My name is John and as I write this, I am full of venom and if care is not taken, I may have to take the laws into my hands and kill someone.
Not just anyone, but my brother-in-law; I mean my elder sister's husband. My sister Jane is the nicest woman any man can have for a wife but her husband, Kenny, seems to take advantage of her calm and quiet nature.
When they were dating, he was very nice and seemed to be a loving and caring man but barely a month after their wedding, he beat her up so bad she was hospitalized. Our family intervened and he pleaded and cried, blaming the devil for his actions.
But since then, my sister has not enjoyed the marriage as she is in and out of hospital and has lost two pregnancies due to severe beating by the brute. But she has remained stuck to him and anyone who suggests that she gets away from him becomes her enemy.
But I can't take it any more. I feel like meeting Kenny man to man and beating the daylights out of him. But I fear my sister would naturally takes sides with him. How do I stop him from killing my only sister? She is the only one I have in this world as our parents died when we were young. How do some men feel beating their wife like a house girl. It bagfuls me honestly.