From what I was reading about RAD, what she wants is for everyone around her to be miserable, and that is what she is getting, so that makes sense. Not reacting is the right reaction, because her goal (seemingly) is to disrupt the entire family and play people against each other.
It's not a good thing, but I'm relieved to know that 1) this is not an uncommon thing for someone with her background and 2) I'm not crazy or imagining things.
For years the only advice I have gotten has been that I should be the adult and have more compassion.
I have compassion, but the people who offer this advice obviously do not live with someone like this. Compassion can be beaten right out of you over time.