How to handle a sensitive information shared by a student

I am a teacher who has set an "open door" policy with her teen students. Ever since I decided to do that, no story shared by them had ever come close to the one I heard recently. A female 15 yo student shared that she is engaging in self-harm, an information I have known about in quite a while but couldn't do much about it since I promised I wouldn't tell. The surprising reason behind that self harm which came up in a recent discussion after i convinced her to search deeper behind the causes of her emotional state is that she had some sort of incestiuous encounters with her little brother when she was 13 and he was 12. Something that stopped suddenly last year after she pushed him to stop whatever it is he was doing to her. I am very lost when it comes to how i can handle such a devastating fact especially that I promised I wouldn't tell anyone. The counselor at school isn't available at all times, doesn't help much according to the stories shared by other students and we live in a pretty conservative country in which such things wouldn't be taken care of properly. The student's mom is pretty strict and her relationship with her daughter isn't that good apparently. Should I alert the administration or call the mother? Or would that make things worse? For now I asked the student to talk to me in the upcoming week again so we can have another in depth discussion about this, especially to help her stop self harm. But i am not a professional and I am afraid that i wouldn't tackle such sensitive matters the right way. Help please 🙏