Continue to do what you are doing. A GREAT JOB. You can't replace the BIO mother nor should you feel that. There is nothing wrong about also approaching the child and phrases her about her soccer game.
The BIO mother will always come frist regardless of what you do, it's learning to step back when needed. But you don't have to step back from the child just allow room for the Bio mother to be first in the child's life. It's not easy when you have developed strong feelings for the child, she lives with you and you have taken the role of mother to her - She is still very young, but she can also see how very loved she is by YOU, her Bio mother, and her Bio father and that is the most important thing. When she becomes an adult she will tell people she has TWO moms.
She will be proud that she was raised by two good woman (regardless of how the bio-mother is or what your thoughts are on her).
If this still bothers you, think of your step daughter as your niece? Would you not run over to your niece and give her a big HUG after a winning game, a school accomplishment, or dry her tears when she's hurt? I love my neices as if they were my own daughters - of course they are my sisters girls but I am a very speical person in each of their lives, when they do well in College they call me, when they have boyfriend issues they call me, when they need something they call me - I feel like I have 8 daughters instead of 8 nieces.
As I have mentioned before, being a step parent is so hard, not knowing your boundries or how to cope when you fall in love with your step children and really want to be their Real Mom....SO to keep your sanity or my approach as I have said is I treat my step children as I would treat my own but think of myself as their Aunt.
(BTW, because of the jealousy of my step-daughter's mother and the relationship I HAD with my step daughter that tooked 4 years to develope and two years going great, she has sabotaged our relationship and it's been 4 weeks since I have seen or heard from my step daughter!) It hurts, I miss her terribly and I'm trying to deal with it one day at a time.