^Which explains a lot about why the sister married that dick in the first place.
If you want to help, offer the sister some support if (and, with any luck, WHEN) she decides to leave his sorry ass.
This is what I would have said.
While it scares me that people like your sister in law sometimes choose to stay with assholes because putting up with abuse is better, in their minds, then being alone...well, there is no way to make that choice for other people. Let's hope she can see the way out of this!
But good for you for caring so much, and being there to support her! Keep reminding her that what happened wasn't her fault and that she deserves better!
Sorry to hear you're all going through this.