You are a great dad. That girl is lucky she had you in her life.
If you were her bio dad, then you would adopt what we call parallel parenting. It's treating each home like a separate kingdom with its own laws, own army, own customs. Only if they encroach on your borders or do harm to one of your citizens (the kids) do you seek legal recourse.
But you are her step dad, so there won't be any custody order. You'll have to learn some very creative tactics for staying engaged in her life.
If I were in your shoes, I think I would read everything in Lesson 5 about raising a resilient kid -----> and maybe some of the material in Lesson 6 on parental alienation. Your stbxw is likely going to deploy some of those tactics. She probably sees her D13 as an extension of herself, and therefore if your wife splits you black, she will expect D13 to split you black too. If D13 doesn't go along with that, she risks being blasted in one way or another.
One of the most valuable things I learned (wish I could remember where it was from) is teaching kids the difference between lying, keeping secrets, privacy, withholding information, and forgetting things. I had to do this with my son (11 at the time) because I didn't want to ask him to do something morally ambiguous. Even though he needed to do some morally ambiguous things. For example, I didn't want son to tell his dad things about what was going on in my life because that put me in harm's way. I also didn't want S13 to think he was lying. When you do this with kids, you help them see how these things have specific contexts. Sometimes it is important to withhold information. Some things are private. Some thing are secrets. And of course, sometimes, things are lies.
It's also a way to teach kids about boundaries in a concrete way. So I would bring up scenarios and then ask S13 if it was a case of lying, secrets, privacy, etc. Sometimes it was complex, and a few things were going on. He liked this exercise for some reason. I didn't tell him why I was doing it (private
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Your SD13 may need to figure out creative ways to keep you in her life. If she is fearful of her mom's rages, then help her figure out how to run the gauntlet so she can minimize conflict with her mom, while still having you in her life.