ouch! that would devastate me. I'm sorry you are being threatened by that. could it have just been her feeling overwhelmed with everything and being unwilling/unable to do what is needed to keep your dog healthier?
I had a similar experience when I first went away to school. I had adopted a dog during high school who had some issues. I was able to deal with them, but when I went away to school, she was left with my mom. One day, my mom became fed up and kicked the dog out of the house without a collar on and refused to let her back inside I'm hoping animal control picked her up and she was able to find another home. My mom did not tell me about it until weeks later, because she wanted to make sure the dog was gone... She now denies ever doing it.
If you want to try striking out on your own again, could you take your dog with you? Or have a conversation with your parents about how manipulative that sounds/feels/is? Maybe pull then into a therapy appointment to address the topic (if you go to therapy)? ((hugs)) I'm sorry you are so trapped by everything.