I am so sorry you are experiencing this...as a mother I hope my children do not fall into depression and SI as I did. all you can do is give her love and support, it might not always be easy but understand that most times the urge is so overwhelming she can't control it.. talk to her about alternatives and let her know she can come to you when she feel she is losing self control. I wish my mother was as understanding as you are...when my mom discovered my cuts she beat the crap out of me I know now she was scared and reacted in the only way she knew how but it didn't help. I started cutting at twelve and sought help at 16 I stopped cutting at 18 and just recently gave in to the urge again at 24...I'm currently seeking help....let her know of this forum and encourage therapy but don't push it.. you want her to feel that she can trust you. However if you believe her life is at risk, maybe she is cutting to close to major veins then do what you have to do as a parent.
Feel free to message me if you ever need to talk or vent... we are all here to listen and support each other.