Seastar This is a bit long so you might want to get a cuppa before you start reading!
I think the most important and urgent issue is whether your sister (and her children) is/are at risk. You must find out, either by asking her directly or by speaking, discretely, to family and friends. This is urgent.
Once you have this information you will have a clear idea of what action, if any, you should take.
I'm afraid I don't remember if I read any of your past posts but I get the impression from others on this thread that you have 'history' regarding your relationship with your sister. Nevertheless, given this upsetting behavior of your "sister's husband*", and regardless of any 'history' you have with your sister, her safety has to be your primary concern.
Whichever approach you use, hopefully your inquiries will prove, without a doubt, that there is no risk, no action needed, you can relax.
If, on the other hand, there's the even the slightest hint that your sister and any children are at risk, then you need a plan. You need to act quickly and you won't have time to be indecisive. It would be a really good idea to ask yourself these questions as soon as you can:
1. Would it be better to distance myself and pass the whole matter over to a more impartial person?
2. Would I be at risk if I were involved?
3. Can I deal with this on my own?
4. If I need advice/a second opinion/help who would I approach?
5. Should I consider approaching an organisation for assistance rather than a friend or family member?
6. Have I looked into any organisations who could either advise me or even take appropriate action?
If you can answer these questions now then you will be well prepared for your next steps and act immediately should it turn out that your "sisters husband*" is indeed an abusive a***hole.
I hope this helps a bit.
*I note with interest that you call him your "sister's husband" rather than brother-in-law. Would it be fair to suggest that you do not have a 'brotherly' relationship with him? 