Advice on my situation

Elaborate, I'm inflicting pain or I should be inflicting pain, or maybe I just look psychotic

The latter two. Your sister is a b'itch, mind my language.

This was a very, very long, but also interesting read.

Your sister is a spoiled brat of the new, broken generation.
If she treated me like that, I would throw her under the bus after one day, having her face consequences.
I wouldn't give a pineapple what my parents thought, even under their roof.
I am amazed you've made it until now, and I admire your patience.

Reading your latest update made me happy for you.
Yiur dad is finally realizing a thing or two about her - he should give her hell; grounding, control of bank account/paycheck money, stuff like that.
No reason to be firm with her, let it rip.

I want to tell you that you should've reacted a long ass time ago, but you've played your cards right, despite all the frustration.
I hope things work out for you, and that your sister will become less of a retard.