You'll obviously need quite a bit of time and talking to process this, but don't forget that even if their entire family unit all have the same narrative, then it doesn't make it completely true. My entire family believe me to be awkward and argumentative. Not one person who knows me away from the family thinks of me like that. However, my family bring out the worst in me so around them I do revert to being a bolshy teen.
It sounds like your dad was very fucked up, and didn't mind if he spread pain so long as he got what he wanted. You know that you and your mum aren't slags, that much is blatantly a lie. He probably DID tell his 2nd wife that he didn't care about you, but also told anyone down the pub how much he loved his kids.
Your step family will all gang together on this - it probably makes them feel better. You still had the wonderful mother that you remember, who did so much for you, and they can't change those memories. And, once you've processed this, you are able to walk away from them, but they still have to live with themselves.
What your step bro did was horribly nasty and intentionally painful, but I suspect that your childhood was considerably happier than his.