So sorry to hear this! I hope she recovers and is able to identify and testify against them - the best thing you can do is be strong for her and tell her she did not deserve this. While she is a victim, she needs to rise above "victim" feelings and defend herself through legal means. At her age, she is at an age where things like this can affect her sense of self-worth in a negative way - your job as a big sister (and for those around you) is to counteract that and make her feel positive about herself - she's going to have an emotional battle that will be a lot longer than her physical recovery. OK, I'm getting off my soapbox now.
I hope and pray that these boys are caught, get what they deserve and that she makes a full recovery physically and mentally, maybe even becoming stronger in the process.