1. Why didn't W know that D14 was sick? 2. Why didn't D18 stop W from wailing on her little sister? 3. Why are you stepping into W's shnit again and trying to rescue her from herself?
What if you were to merely answer "oh?" to her declarations of leaving? She's trying to get you to play the game that you have always played, namely that you will reel her back in and thereby assume the responsibility for what happened in the first place. My advice is to say nothing...do not offer to help her pack, do not beg her to stay, simply be willing to listen to her tell you of her plans and otherwise avoid contact with her, unless it is to politely inquire about her progress on leaving the home.
While your D was out of line in kicking the dog, her rage is certainly understandable. I think the greater crime here is that SHE was abused by her own mother. I'm sure that your local child abuse authorities would agree and would have D removed from the home.
