I think she still needs you aswell so dont forget about her as you said, it is very natural and a good thing her father came back into her life, she needs you aswell ,its hard as you think she doesnt care for you anymore,I doubt if thats the case, once she gets used to having him around some of the newness will go and you will both be in her life ,its a wonderful thing she has you both.
How is she your step daughter? You are not with her biological dad? I'm confused as to how you wound up raising her?