Report it to the police, and then do everything in your power to try and convince your sister to get a restraining order.
My cousin was also in a physically abusive relationship years back, and she would often wear turtlenecks, and hide the choke marks on her neck.
The guy was a complete asshole, and had a grip on her that I've never seen. The breaking point was when they got into a major car accident, and my cousin's scalp was torn off, and she ended up requiring almost a mini facelift in order to seal up the wound. And what was worse was that it turned out that the guy was fine, and didn't want to get in trouble for the accident, so he dragged her beneath the driver seat side (the car was upside down), and then claimed she was driving.
It was only after this incident, she finally got a restraining order against him, but even afterwards, it still took time for her to get over him.
A restraining order will be important because it was one that the parents, and she had on him, and since she moved back home with them after the incident, he wasn't allowed to get anywhere close, and when she called him, he wasn't allowed to pick up either, as that would break the restraining order, and eventually, he just disappeared from their lives.
Don't let it get to this kind of a breaking point.
Do everything you can to try and step in, especially if there's plenty of physical evidence to prove that he beat her like that.
Report it to the police, asap, even if it's just to get a report on file, because you need to get it started.
Best of luck to you and your family.