I see quite a few people end up here after searching things like "Step Parents and RAD" so I figured I would put something out there. I don't envy you. You have one of the hardest, most thankless jobs known to mankind. I've watched my wife go through it. You want to love them but they will do everything in their power to thwart that. They don't want it - there is no Love, only Pain. And they won't let you forget it. But I don't want you to think it's hopeless. It's not. But you're going to have to earn it.
My wife and oldest daughter started off with a really rocky relationship. For the very reason above. My wife loved little girls and did her best with my daughter. She tried reading to her, she tried making things with her, she tried anything to bond. But it just wouldn't happen. For years she tried, only to meet resistance time and time again. And it wasn't just passive resistance - my wife was made out to be the true evil step-mother / witch that wouldn't think twice about sticking children in ovens.
Which hurt. A lot I imagine. But she didn't stop looking for opportunities to bond. For years she seemed to get nowhere.
But after years of hurt feelings, loss of trust, physical fear, and utter disrespect, things began to change. For the better. My daughter began to show signs of respect for her step-mother. With time, those began to show more and more. They began developing a rocky and frequently adversarial relationship, which became less and less so over the next year or two.
I can't tell you how many times my wife threw up her arms in frustration or disgust and screamed "I can't do this any more". But more importantly, for each time she said it, there is also a time when she stayed. And kept trying.
I've spoken with other parents where that didn't happen. They had it bad up until the magical age of 18 no matter how hard they tried.
But there is hope. I know there is.
As for my wife and her step-mother? Today, they have moments where I'm almost jealous of how close they
can be. It doesn't happen a lot, but I love it when it does.