Look at this from your step-daughter's perspective.
Her mum and dad's marriage - the supposedly indestructable bedrock of her short existance has come apart, and Daddy loves someone else. She is going to find it really hard to find a way to love you - quite apart from your feelings for her!
I do know what I'm talking about - my two eldest daughters are my step-children, and we have a third daughter as well. They were six and four when I appeared, and it was a difficult adjustment for everyone.
You need to approach this relationship carefully. On no account try to pretend you are her 'new' mum - but make it clear that you make her dad happy, and you want her to help you with that - get her on your side.
Be affectionate when she wants to - but give her space when she doesn't. Follow her lead, and try to spend some of your time with her just with you and her, doing girly things like shopping, or cooking, even just watching TV, to give her a feeling that this is natural and relaxed.
Children of this age are adaptable, but remember, she may hear conflicting information from home, but don;t force the issue - prove you are the 'good guy' with your approach and attitude to her.
The fundamental rule is that you and your partners must back each other up. If she asks you for a sweet and you say 'No, dinner's ready ...' then dad's answer when she asks him (and she wiil!) must be along the lines of "Pink has said no, no point asking me ...". If she can divide you, she will - that's not her being spiteful against you, it's what children do - biological or step-children, that's the way they behave. What you have to do is not fret from an adult perspective, just establish and keep the rules, that gives her security, and she will gradually come around to developing a relationship with you.
ctd.