This can be tough on the family and being her brother I can understand your concern. But truth be told, you cannot do much to help them. Usually with couples, it is effective only when they try to resolve conflicts.
If you would like to help, try to give your sister the courage to stand up to herself. If she is right she should stand up to herself.
Next Steps
Please try and get the families to talk about this. Or advice them to go to a marriage counsellor so they can see each other's point of view.
Health Tips
Marriage problems are a bit tricky, all you can really do be very supportive of your sister.