I'm in the same situation, except I'm an aunt and it's my sister who's doing this to my niece (with whom I share a close bond and love as though she were my own daughter).
I came to this site in search of the same questions you have asked.
I find some of the responses to this thread to be quite dismissive, as though your word and intuition about your step-daughter's change in behaviour is being imagined. But let me tell you, your post struck such a chord with me as I'm experiencing exactly the same things. Unanswered phone calls and messages, no longer comes to visit me. In my case this change happened overnight, after my sister cut off contact just because I challenged her for making a false allegation against me (a petty issue, unconnected to my niece). So I went from regular contact with my niece every week and being a big part of her life, to nothing.
In my case, I know that this is being caused by my sister alienating my niece, because my sister informed our mother that she was breaking off contact.
The alienation started in January. I managed to see my niece around a month after it started. She was happy to see me and we shared hugs and kisses. We chatted that day, like before, the same affection as though nothing had happened (her mother wasn't there). There was an elephant in the room (the issue of blocked contact) but I didn't want to force the issue, since my niece seemed to be fine and happy in my company. We agreed to contact each other via messages on a certain platform, but after that day I heard nothing and she stopped logging on to that platform.
This week, I managed to see my niece again (she was with her mother, and her grandmother [my mother] was also present). I called out to her so we could chat alone (my sister pulling a face) and my niece actually running away from me. I went after her and she eventually came with me but she didn't say a word to me, just wore a grumpy expression like she didn't want to be with me. The only communication I received was the slightest of nods when I told her that I would always love her.
I was concerned about the deterioration since I last saw her, but what was more worrying was my sister then coming up to me and accusing me of causing my niece to suffer a sleepless night (the day I had seen / spoken to my niece without her mother present). So my sister obviously used some form of coercion, or threat or manipulation to make my niece feel that way, because she had been happy when she left my company.
I'm torn between taking legal action (for a child arrangement order) to restore contact but a part of me fears that this will make matters worse for my niece, as my sister will undoubtedly double down on what she's doing. Furthermore, if contact was only resuming because of a court order, I fear that my sister would make the contact become an unpleasant experience for my niece, so that our happy memories together would be replaced by unpleasant ones.
For context: My mother and sister have a golden child / narcissistic mother dynamic, and I believe this is what my sister is trying to replicate in her own relationship with her daughter. My sister is highly manipulative, which is something I've had to deal with my whole life, as the scapegoat versus the golden child. So I'm very concerned about the impact this will be having on my niece and also whether this is likely to distort her views on what a healthy relationship looks like, and whether she has the defences to cope.
I was trapped in that golden child / scapegoat dynamic for most of my life, and I was gaslit by my mother and sister (both supporting each other) to believe that I was at fault or flawed in some way. They still do it, but I recognise it for what it is now, so I can call it out. At the same time, I know how badly it affected me at various points throughout my life, and I don't want that for my niece.
I know this is an old post, but I hope your situation improved OP.
Also, I would be interested to hear from people who were alienated as children, and the same questions the OP asked.
@btux