IMO your sil doesn't need any more pressure than that she's already under by continue to live with a violent twunt and I suspect this isn't the first time she's given him a 'last chance' and nor, unfortunately, is it likely to the be final occasion she does so.
My fear is that if you approach your sil, she will effectively go underground - hiding/camoflauging her bruises and making like everything's fine, he's getting help with anger management, he's a reformed character, etc.
Notwithstanding the dv she's enduring, you bil is the one with the problem and he's the one who should have the screws applied to his tiny balls by being told in no uncertain terms that what he's doing is known to you/your dp and that if you have any reason whatsoever to suspect that he has not seen fit to restrain himself from using violence on his dp, you won't hesitate to approach the police, social services, health visitors, his/her GP, Uncle Tom Cobley, and all including his neighbours, in order to ensure that his dc does not grow up in a toxic environment.
Tell him that if he moves his family elsewhere you will alert all the relevant authorities and that you will do the same should he attempt to prevent you visiting your sil.
In other words, give him no quarter in your efforts to make him realise that his violence has to stop, otherwise the only contact he'll be having with his dc will be subject to ss supervision.
As it very much sounds as if your mil is part of the problem she may not be the best person to raise the subject of his violence with him but, neverthless, she needs to know that her dgc is at risk of history repeating itself if for no other reason than the more people who know what he's doing, the more chance there is of him being caught in the act or shortly thereafter.
Tell him that as he enjoys beating his dp up, he should take up boxing and get the shit knocked out of him focus his anger on an actual punchbag or on men his own size rather than a woman.