I am so, so sorry NuggetBrain. That whole book thing that your stepmother's sister did is crazy. They clearly don't care about your dad at all. I am sorry you had to sit and and watch that.
I am sorry. This is when your walls need to be strong and you go in doing what you want and eff everyone else. Take care of you, your dad and nephew the best way you can. Let all of the BS slide down your back like water off a duck.
I know it is hard, but you will be saner for it. I speak from dysfunctional experience.