It'll be tough for you at first, but lock yourself in your room and starve yourself for a day. She will start caring - (I hope anyway). If this is successful, use that as a weak spot in your argument.
"You would only save me at the risk of my death, yet you will spoil and lavishly treat 5 horses and my sister to whatever they want. For all you seem to care, put me in a shoebox on side of the road and don't feed me. I am your son, not a sub-human. I do the work, and should be entitled to my rights. Without me, this family would be in shambles."
Record the entire conversation just incase she doesn't care. If she gives you crap, she gets hell. It's reportable to the police I believe.
