Well she's currently in her bedroom like nothing happened, doesn't k ow or care what she's done to her sister. In her own little bubble
It can be so damn infuriating when your kid is acting out, and two minutes later seems to be fine and dandy while you are still reeling. I get that.
But, just because it appears that she is fine, doesn't mean she actually is.
If she has been through a lot then can you not appreciate that the anger is coming from a place of hurt, not coping, maybe confusion?
Three years ago she was 13, the younger ones were under 10. Its not a far stretch to think that whatever happened affected her more because she was more aware of all of it.
The way you talk about SS is so dismissive...I have met a few officious pricks of social workers myself, but even aside from the nonsense, they had a point. You seem so defensive I think its just your default to instantly feel attacked and lash out. But people are trying to help, if only you would listen.