Oh, man, I have tears in my eyes, Sarah! How can anyone do that to someone?!
Seriously, go to the media. Go to the police and make a huge fuss as to why they haven't taken those tapes from the store as well. The police were useless when my nephew was jumped too (I mentioned it in your other thread here). The circumstances were different with him and we didn't want to involve the media and make him a further target, but it was so frustrating. My brother asked them, "Aren't you the least bit concerned what adult non-students are doing hanging out on schoolgrounds? Why are they there? Are they selling drugs? Picking up young girls? Isn't that something you want to know?" They just shrugged and said they don't have the manpower to be at the schools all the time. When my brother pressed the issue she used the excuse that, "Well, your son did shove back." Um, hello?!? Should he just take his freakin' beating quietly without any attempt to defend himself? And how do you know that he shoved back? Because he told you! Why would he tell you that if he had anything to hide?! Grrrrr, so frustrating.
Like I said, we didn't pursue the media, because we felt it would've been more horrifying for my nephew than the actual incident, and likely would make him more a target than help the situation. But in your sister's case, the media should definitely hear about this. Get them focusing attention on that store and how there's been more than one attack there and the storeowner doesn't want to cooperate. Put some heat on him and on the police to do something about it. I swear they see that it's teenagers and assume they did something to bring it on and it's so unfair. Don't let them get away with apathy!