I guess only you know how you feel about your step-daughter. Weigh up how you will feel if you don't reply to how you will feel if you do and it's not genuine. What's the worse that can happen if you reply and she's doing it for her mother? You will feel upset, and it could drag you down again, but you're a strong person who can get through things. If you didn't reply, you could miss out on a whole father-daughter relationship, and in the future a grandad role, could you live without that?
I guess if it was blood, and she was close to her mam still, you'd probably have less reservations, but it sounds like you're scared that she's your stepdaughter and not your 'real' daughter - do you feel like you're her real dad? If you were there for her from a young age, it's quite possible that she thinks of you as her dad? Maybe she really genuinely wants a relationship with you, even if she is close with her Mam too? I can be done?
You must feel something for her, or you wouldn't ask on here, and would just ignore it?
I suppose what I'm saying it that only YOU know how you feel about things. Maybe you're too scared of getting hurt, which is quite understandable.
Also, have I got it right in that your wife was violent towards you? Have you thought that maybe she was violent towards her daughter as well, and that could be why the daughter feels like she needs you in her life? Maybe she didn't tell you as she was scared of her mother? Played a perfect daughter to keep her mother happy?
I have to say though, that you're a very brave man to talk about this and you're so strong. You've found the courage to leave and you're doing so well for yourself - you should be proud that you're building a new life and doing so well. x