My cousin cuts and wants to kill herself... help!?

Her name is Samantha and I love her so much. She's really sweet, nice, fun, and you get the idea... well, I found out she cuts, and she really badly wants to kill herself with a knife... she told me a couple weeks ago she was in her room and she was about to slit her throat, but her brother, Jonas, walked in on her so she stopped. I don't know what to do... only her closest friends know about this, and me... She said that if I told any adults about this, she'd be really angry at me, and she said that she can deal with it and stuff... She promised me that she'd only cut, but if she accidentally cut too deep, then thats it. She said that i'll forget about her like everyone else will.. I don't know what to do... please help..