For us it's hard to understand hurting yourself to get rid of pain. For them it's easy. They need someone to talk to...in my experience instead of sharing what was wrong they hold it all in. Their only release is cutting themselves. It is very scary to us but not to them. We feel so helpless as to what to do because it does scare us so much. We want to immediately fix what's wrong but it takes time. She needs someone she trusts and can talk to...counseling can help. But don't be surprised if it doesn't help right away or takes a long time to work out all the problems. Lots of times depression or anxiety plays a huge role in their cutting....before a stressful event (maybe a test) or when someone has really hurt them or upset them.
This is only my personal experience with a cutter. It can be different for each person. Sending up prayers!
Also...hanging out with people that are a good influence helps. If her friend is doing it too then your sister needs to talk to her family. Best wishes for your niece and strength for your sister (& you) to figure out the best way to help her.
Tracy H. #31511=)
"The beginning is always today!" Mary Shelley