I’ve spent a few years among families of Rabbonim who teach in kiruv Yeshivos and bring these off the derech boys home on a regular basis. I would say that because their children understand that they are playing a part in trying to help, they tend to objectify them rather than learn from them, often referring to them as “the bochurim”. All the while these kids will happily play with them.
So maybe you can explain to the rest of your kids that daughter X is going through a difficult time but we all really want her to get better so let’s show her extra love by doing xyz. In this way maybe they will feel that they are part of the helping initiative and distance themselves from her actions while still showing her love.
Also, your daughters actions as you describe them really show that she is doing it out of anger. Eventually her anger will pass as she grows up and what will happen then can be heavily affected by your actions now. I definitely agree with Shrek that she is testing the waters. Show her your unconditional love now.
If she is still ‘talkable’ try to have some mother-daughter time once in a while. Maybe just the two of you going out to eat. Discuss normal things with her, instead of chastising her or telling her how pained you are. Tell her that no matter what, you still want that relationship with her above anything else.
If she is not receptive, then take the initiative. If you know she will be going out one night, leave her a peckel of her favorite food or treat on her pillow or in a place where she will definitely get it. Leave her small things, just to show her that you think about her, but don’t want to intrude into her space until she is ready to talk.
Remember that above all, she is suffering from pain and anger that she doesn’t know how to deal with. Your love can help her more than anyone else.