Our boys also have sisters being raised by their birthmom. That is just what they call them ... their sisters (lol sometimes). I was very proud of my oldest son this past summer when he mentioned to someone commenting on how we only had boys in our family that he also had two birth sisters.
The reality is that your daugther DOES have a brother. Calling him "someone special" will diminish that relationship to your daughter. I KNOW how hard that is in the beginning when you want your other daughter and your new daughter to feel like sisters. But it DOES get easier - and this is the reality of your daughter's life -she has brothers and sisters she doesnt live with. I think thats ok to share
I promise, it will all feel more comfortable given enough time.