It's difficult for me to respond to your question without putting it through the lens of my own experience, and I realize that no two situations are exactly alike, and mine may be of little relevance to your own. That said, perhaps I might share a piece of my history with you?
My father and step mother were married when I was 13 years old. My mother never spoke negatively about my step mother, nor their relationship. In fact, I think she was relieved when my father found someone to share his time with. Still, I had 'sided' with my mother, as much as a child can. I was angry with my father and believed the divorce was his doing.
At first I liked my step mother very much. I was happy for my dad, helped make wedding decorations, and would go on outings with my SM. She was open with me - too open - sharing grievances she had with my dad, and in a juvenile attempt to protect her, I opened up as well, urging her to stick up for herself, communicating a history with her that was biased from my own perception.
She in turn shared these things with my father, and I felt terribly betrayed. My papa is reactive and volatile and accused me of 'black mouthing him'. For many months, he disowned me.
When I started going back to his house, I despised my SM for breaking my confidence and inserting herself between my father and me. When I look back now, I see this through the eyes of a parent and adult. It's more complex than that - they were where they were at - and I don't think my SM was necessarily trying to sabotage me or my relationship to my dad. It's taken a while for me to get there, and I had to get there in my own time. When I was a kid, I couldn't see past my developmental capacity. Teenagers are valuable human beings, and also lack certain abilities that they develop as a result of their teenage experiences. I had a limited perspective and my folks punished me for it. I wasn't doing it to be difficult, it was just a natural stage in my progression.
After the upset, I wanted little to do with my SM. I no longer trusted her. Instead of just letting me be for a while, letting me be upset and slowly building trust back, my father and SM pushed themselves on me. I wasn't allowed to have alone time with my father, wasn't allowed in their home if I didn't accept my SM as my family, wasn't allowed to be angry. They pushed and I drew back farther. Unfortunate, because the wounds lasted for a decade - and I think they were unnecessary, should the adults in the situation just held space for me, instead of trying to force me into their idea of what should be.
I can relate to the difficulty of being pushed away by an angry child, I myself raise a kiddo who isn't biologically my own, and, as is normal per her developmental age, blamed me for having lost her family unit. It really hurt, trying to give this hurt child love, and getting attacked in response. I just had to hang tight, love her for the both of us, not rush relationship, and let her be as angry as she needed to. She's much younger than your step children, and it's not the same situation I know, but that's what worked in my home. When she through insults at me, I would let her know that I heard her, she was feeling angry and missed her bios. I let her know it was okay to feel upset, but it wasn't okay to be rude to me. I tried not to take it personally, but sometimes the night would come and I would cry myself to sleep. But around her, I didn't put that on her, the responsibility of my emotional well being. I'm an adult, it's my job to take care of me, keep myself sane. Not hers. She's a child. She's allowed to be a child.
I hope the best for your family. It's hard having strife with the people we love most.