Family issues.

Azkadellia • Currently just trying to get my body to do what it`s supposed to. Lesbian.
**This is a long post, with swear words, partly a vent, partly asking for advice. Please don't be rude about it.** So, when I was about 10 my dad got remarried to my now Step-mom, at first she acted like she liked me. She was trying to be my best friend or whatever, but I noticed that she treated my older cousin (who was about 17 at the time) like complete shit. Well, I just passed it off as my cousin not liking her very much, because to be honest no one in my family really likes her. 
Anyway, fast forward 6 years, my cousin has been moved out for years. And now, she's started treating me like shit. She and my dad have two kids, and she definitely favors them over me even though I was in her life first. Fast forward another 2 years, I'm 18 now. My oldest little sister is 7 almost 8, and that little girl likes to push my buttons and piss me off.
When she irritates me I (jokingly) tell her I'm gonna punch her in the face, if she doesn't stop. I've asked her about it, she doesn't mind she'll literally just laugh at me. My step-mom however has recently been screaming at me her she doesn't like it and I need to stop. In my family, the older siblings always joke around like that with the younger siblings, but since she's been around I haven't been allowed to do anything my older cousins (who my dad raised and are line my siblings) did with me.
She's constantly threatening to call the cops on me for the stupidest things, she says I'm lazy because I don't have a job, yet I can't get one because she's constantly using me as a babysitter for her kids even when I have other plans. When I try to decline, she screams at me that I'm an ungrateful little bitch who never helps out. I've had two jobs, both that had to be quit within two weeks due to her not wanting to pick me up from work, and the busses not running late enough for me to take a bus home. 
She's forever yelling at me, yelling at my dad because of me, telling him I'm a problem and a useless waste of space. I do my best to help out around the house, yet it's never enough. Her most recent complaint, is that my dad lets me run the house and she hates it when I correct her kids, (even though I'm with them more than she is.) 
So that's been a huge source of tension, I have no other place to go, no money saved, no car or anything. My dad always swears he's going to talk to her, but he never does. He just lets her continue with this ridiculous hateful behavior. And tonight, I blew up, and started screaming and crying because she yelled at me for telling my little sister to shut up because I was in the middle of a sentence.
I told her I was tired of being disrespected, and that she needed to control her kid because I was tired of both of them treating me like shit. Now she's hiding in her room crying, and posting hateful shit on Facebook. Do you guys think I went to far in trying to defend myself? I've been dealing with this for years now, and I would be moved out by now if I could afford it, but as it is I can't get another job due to my first two having to be quit without prior notice, no one wants to hire me.
I feeling so stuck, and just awful. I hate fighting with her, because she was very helpful and nice to me when I needed it when I was younger. Yet, now she's just a horrible person, she clearly favors her kids over me. Even told me her oldest was her favorite, and I just don't know what to do. Why advice?