Not necessarily true. She could just get freaked out after she does it and shows you as a cry for help. People who have never cut have their own misconceptions about why someone cuts and/or why someone cuts then shows someone else that are sometimes true but not always true. It varies from individual to individual.
Like I said, don't treat her any differently. That includes not giving into what she wants more often than usual.
What's surprising she cuts were its visible the doc at ER said she's using it to manipulate us she gets what she wants or she cuts then shows us what she did
I so sorry to hear about your niece. I used to cut myself, my thighs that way no one could see. You cut because you have mental pain and in my case physical pain was better. At least you could feel it which would relieve the mental pain. I was never admitted institutionalized but I did see a psychiatrist and between that and medication I have come to learn how to deal with my mental pain and no longer cut. I no longer have or feel the mental pain. I pray your niece the she recovers from this hidden problem.
She needs to have some counseling that discusses the cutting and why she does it. Once she figures out why, she can begin to recover. It will always be a struggle for her, but she can at least learn techniques to use instead (i.e., taking an ice cube and holding it until it's completely melted). You can choose to take her to a mental hospital for inpatient care if you have truly exhausted all out patient options and tried more than just one therapist (it may take a few to find one she clicks with) or she is a danger to herself or others.
Keep in mind not to judge her and not to assume she's trying to or will try to kill herself just because she's cutting. Cutting releases endorphins which feels good. It's physical pain to match the emotional turmoil inside. Some people also do it as a cry for attention, but not everyone does it for that reason. Treat her like normal, don't go about checking for new scars, etc. It'll make her withdraw more if you act differently and if you check, she'll just find more "creative" places to cut. The problem needs to be treated, and most likely, the cutting is not the problem. It's a result of the problem.