I need help with family issues?

My father, for all his brain power, tends to be fairly oblivious to how he treats others. If he feels he’s correct in an issue, there is nothing anyone can do to show him otherwise, and he will get as ugly as he has to, as if it’s his duty to see this cause through at all cost.
His latest ‘theme’ (he goes through them) is that my sister is lazy and worthless and needs to be whipped in shape. I’m scared that this is partly my fault, because I’ve been all over the place in behavior, openly hated him, now quietly tolerate him, have almost flunked out of school, etc. I should also mention his greatest fear is me and my sister will turn out “lazy failures” as he thinks his sisters became.
He doesn’t see it this way. I know that. He sees it as him being a good father, and getting on her for attitude.

My sister is very quiet, and takes it all in until she can’t anymore. Honestly, her “attitude” is normal 16-year-old not-unquestioningly-obeying-parents. She will help, but if she’s being demanded, she’ll make it difficult. The only time she gets lippy is when he starts in on her. But he sees the lip then, and it’s ‘hah! it’s there!’
(She’s also just started summer vacation, after a very emotionally and academically draining year, and any decompressing she does, he sees as selfishness, any work she does helping my mom around the house, he sees as ‘damn right that’s what you do.’)
(Also, he’s always sort of been the cause of insecurity she’s had to work through; like, when she was younger, he’d tell her she was fat; repeatedly; now she tries to stay as far detached from our parents as possible, because they can be a little stifling. She’s at a place where she’s beginning to blossom, I don’t want to see that crushed.)

She came into my room to get away from him, and at the door frame he rounded in on her. He doesn’t care who’s there when he’s in his righteousness-mood.

I started shaking, no way anyone treats my sister like that. I usually don’t speak up, but there were words so they came. Something to the effect of You’re being a jerk… You’re alienating her… No, you shut up, she’s fine… She is not leaving my room as long as you’re standing there in the doorway… He ignored me and continued to pound at her.
I’ve become sort of untouchable since my failed stint in therapy, so nothing I do ever gets called out. I could cuss him off, he’d blame it on somebody else.

Unsurprisingly, my sister quickly left the room. He continued, yelling at me, but really just so he could be loud enough for her to hear, about how he’s not wrong, no one has any idea how hard he works (me: I think they do, but I think everyone works very hard) and he works his ass off trying to make a better life for everyone in this family when has he gotten to take a vacation, he’s done nothing wrong, she’s done everything wrong.
Slam goes my door as he yanks it closed and leaves.

Now he’s yelling with my mom. Last thing I heard (about my sister) is he’s going to crack down on her, going to take nothing coming out of her mouth. And my mom trying to get him to shut up… but he won’t. He’s upset.

Now it’s that deadly quiet after a fight.

I need help. I’ve made it worse.