You are to be commended for rescuing this child!
I'm not an expert on this and I'm sure you will get many helpful responses either on techniques or on books that can be shared.
I just want to encourage you to keep on plugging away! Whenever I got ticked off at my parents, I assumed I was "adopted" because that meant I wasn't really theirs. What I didn't get is that adoption is the ultimate act of love. Bio kids come to you, but adopted kids are CHOSEN by you. Let your daughter know that she was a conscious choice by your and your husband. My husband is adopted and he has always felt that his "adoptive" parents are just "his parents" - other than some curiosity about medical history and ethnic/national background, he has no interest in his birth family. Your daughter's situation is complicated because of the openness of the arrangement and the fact that she has lived with her birth mom.
I agree that any new baby is going to be a huge blow to your daughter - while it might be "neat" that she has a sibling, if the birth mom is keeping that child, it will increase your daughter's feeling of isolation. Until you know more info about where that baby is, and until the birth mom expresses some interest in seeing your daughter or at least contacting her, I think you have to kind of sit tight and not make overtures. You don't know what you're going to find on the other end, and your daughter needs dependability and stability.
I know there are books about all kinds of families coming together for different reasons. Maybe you can find some to share with her. Even if they represent vastly different arrangements from what you have (grandparents raising kids, people who adopt children with multiple disabilities), it might broaden your daughter's horizons a little, so that her family looks more "normal" in her eyes. Stories of people adopting multi-racial children, or kids with medical issues, and so on, might show her that adoptive parents are truly motivated to give love and care to children. They make this choice, they don't just cope with a stray child that is handed to them.
As for the birth mother being viewed as the "fun one" - this is the same thing many of us deal with when there is a divorce and custody is shared. The non-custodial parent is often viewed as the President of Party Central.
I think you can share with all of your children that you make rules because you love them, that you do all those chores (make meals, clean, shop, help with homework) and so on because you love them and they need that support and guidance. It's not glamorous but it's true love to do the hard stuff with kids. Anyone can do the easy stuff.
Good luck!