How old is your sister?
We are going through a similar situation with my 21 YO daughter. She confided 2 weeks ago that this scumbag beat, choked, and tackled her on numerous occasions. (We suspected this but she denied it). Always told the police nothing happened.
Since she is in college and is financially dependent on me, I used the only leverage I had. Since shame and physical harm was not enough to sway her, I threatened to cut her off financially and put her out of the house.
Since she confided the facts, I have pushed (successfully) for the police department detectives to pursue criminal charges on him. The DA is going to accept the case and ask the court to issue an arrest warrant.
She said she was scared to leave him. Said she feared he would come after her for going after him legally. I told her she has more to fear if she does NOT go after him.
As for your position, find out where the incidents happened. Find out who called the police. If neighbors called, go to the police with that information. If there were eyewitnesses, perhaps the police can go after him. Likely not though as any case would hinge on your sister's statements, testimony and willingness to pursue this.
Call the women's shelter in you city. Maybe if your sister knew what the end of this relationship would look like, she may ask for help before it gets there. The responding police officer was very graphic when describing to my daughter how many women look at the end of that ordeal if they don't get out.
Good luck.