No idea how to go from here

It started with finding the woman that I had been looking for my whole life. She had daughters and I was glad to be a father figure for them. After we moved in together and especially after getting married (18 months after we met), there were some issues with the oldest girl. We mostly attributed this to all the changes and adjustments that came about at once, and tried to deal with each situation as it came up. But then the devastation happened. It came out that the oldest had been doing inappropriate things to and with her sisters, which led to finding out things had happened to her when she was younger. And this was just a month after taking her to an evaluation for cutting her arms. She had to leave the house per the investigator which only ended up being 5 days, instead of 45. I've tried to get my wife to understand that I see there are serious issues and her daughter needs to be somewhere where she can get help. My wife is not wanting to see this as she just "wants her daughter back" my mother and sister who were the ones that the younger sisters confided in, also see that the girl needs serious intervention and shouldn't be in the home. The investigator doesn't believe there will be more than weekly counseling unless the clinical evaluator determines the need, and they aren't expecting it to go the far. But I worry for the other girls because she made threats to hurt them "if they told on her" The police wouldn't even pursue charges because they viewed her as too young. I don't know how to get my wife to understand that I believe that it's dangerous and a bad idea for the daughter to be here, not without serious intense treatment. I fear that pushing the idea further and insisting will just appear that I'm against her daughter now.

She doesn't need punishment, she needs treatment. And yeah, if you force the issue, you will put the wrong words to your wife's mouth. Don't try to scare her into action, fear stops you, it doesn't help you act. You can only be supportive and make them see point with logic, not fear! Try to establish a communication with the daughter, more than what you have. Don't be a father to her, be a friend. Make her sure nothing she does can ever hurt you. Because she is holding on to this ridiculous thought that she is poison somehow. If she is threatening you with hurting someone, trying to push you away, yes this is a fact. Let her drop her guard, by being strong with her, not against her. Being strong as in saying and doing all the things that will not be forceful like trying to control her, but as in calm and assuring. Let her speak up on her own, and talk about what had happened to her. She has to be the one to walk through the door, you can't open the door for her. But you can make sure she does... And this could bring the whole family closer than before. But sometimes logic fails because the person will not let go of the pain they carry inside. She has to face it, but she can't, no amount of drugs or therapy will help her if someone in the family won't show her that pain is nothing to be afraid of by not being afraid of it themselves. She is already on the edge, please don't let her fall off the cliff! Because she wants to!