Hi butterfly2,
You are being a good big sibling by worrying about your little sister. It is really hard sometimes to work out if behaviour is due to "normal" and healthy growing up processes where adolescents are taking on the tough job of sorting out and managing their internal world themselves, or whether they are actually sinking under the weight of difficult thoughts and emotions like depression, anxiety , trauma, bullying etc and not coping.
My view is that you can't be the one to have the responsibility to make that call. I would alert a responsible and caring adult to help you talk to your sister and try to identify what is going on and make some plans about how to help her.
If you have parents that you can talk to, definitely that would be the first port of call. If not, try a trusted other family member or then someone in her school community. A good teacher or school counsellor.
You've done a great job being the one to see a possible issue , but you can't fix it alone. She will never hate you for caring enough about her to try and help her.
If you get stuck , call eheadspace or Kidshelpline for assistance.
