Jesus Christ!!! Miles I think you should show all of this to the police, her notes & updates as well as what you've written!!!! If they won't listen go to the head of your church (if you have one) or the principal of her school or yours!!!!!
I'm so sorry!!! This isn't your fault, your shitty parents put you in a spot you should never have had to be in!!!! To me you sound like you were an awesome caring brother that loved her!!! She was lucky to have you in her life!!!!
Please think about showing all this to someone in authority ok!!
You take care & again I'm sooo sorry for both of you!!!! :'(