my commnt might be useless here and sorry if it will be offensive to anyone. here in my country we have a saying that is "if you need something from a dog , call him master until you get it"
i know it's hard but bare with them so you get to stay at least till collage. your mother stance is just obscene , i dont even wanna call her 'a mother' and i know there might be some kind of pressure by that stupid trash step-dad on her but still ,your her kid.but situations like this however shows people for who they really are , she will need you later in life and when the at time comes it's up to you to treat her likewise or be the better person.
meanwhile , just call the animal master , and as others suggested ,have a back up plan , and remember there's always an option whatever happens there's always a solution . it's just that some options are easier than others thats all
your still so young and have your life ahead of you , learn from these situations , and sooner or later it will be over
my heart is with you and i really hope this all turns into a memory in your successful future life