I absolutely agree that parents should be held responsible for their minor children, especially if they participated. Holding someone down in a beating is a Felony; Aggravated Battery, and although I didn't hear the story, I would hope the women were indeed prosecuted regardless if they were related or not.
I think you will agree that children learn from their parents. Unless there is something mentally wrong with the child, the parent is responsible for that child until they are adults. I don't care if the parent is single, poor or uneducated. They brought that child into the world to live among other people and they have a job not only to that child but to society in general to teach that child certain lessons which include the difference between right and wrong, respect for others and authority, tolerance for differences and empathy. If that child fails and hurts someone, you can't solely blame that child for their actions. Every choice that child makes is derived from the lessons that parent has taught and has shown.
People should always keep in mind that being a parent is not only the hardest job you will ever have, it is the most important. You are making an adult person who will go out and make decisions that may change the world. Nobody said it was easy. If you can't handle the job, then just don't be a parent.
Great question, by the way.