Bio Kid Vs Step Kid

I'm due with my rainbow baby in August. I have a step daughter who is 6. I've been with her dad since she was 2..... I feel awful and i don't care if I get bashed for saying this but I just don't have a bond with my step kid like I already feel with my biological daughter. Is this wrong? Am I wrong? I've tried to be good and build a relationship with her but it just feels IMPOSSIBLE. S