Well your dad's wife is a cunt, pal. A cruel and nasty cunt.
This is going to sound counter intuitive but bear with me, I've a point at the end! I felt so, so, so lonely when I left an abusive partner. Empty. I so desperately wanted to keep the people in my life present so I wasn't alone. But I realised that ending that relationship was actually an opportunity for me to cut loose ALL the people who weren't really fully on my side. Who weren't fully supportive. Who minimised or trivialised it.
And my god I cannot recommend quality over quantity enough! I lost maybe a dozen friends (some mutual with him which is always tough) and went temporarily no contact and then permanently low contact with once I felt a little more resilient again e.g able to attend a family do and be civil then leave when I wanted.
It was liberating to think "you know what? You weren't there for my absolute lowest moments so fuck you, thinking you can bring me down and / or get to be there when I'm better again."
I've told this story before on here but I remember a certain day after the break up, waking up in bed alone and quite literally bursting out laughing. Genuine, proper joy at the fact I was FREE. Safe. No arseholes to manage, nobody else's moods to regulate, no upcoming social do to get anxious about.
And the people who did stay around were angels. And still are. And we've helped each other through whatever life has thrown at us.
I would say, gently, that if your dad is capable of being with a woman who has openly laughed at his daughter being physically attacked, he is not a safe person for you to be around emotionally at the moment. Because he should be someone you can look to for unwavering support and he's unable or unwilling to provide that. Seeing people like his wife will hold you back from getting better. It is better at the moment to feel sad someone isn't there for you and so not see them, than it is to see them and have them making you question yourself re are you overrreacting etc.
Because you aren't.
If you're lonely, please do keep talking on here if it helps. There are some posters who keep us all posted on what they're going through, sometimes for months or even years, and use this as an outlet and source of support and help others in their initial situation when they're a bit further down the line.
You've nearly got another day under your belt! 