Adopting 5year old who I have raised since they were 2. Need help explaining it to her.

I posted this in the r/parenting subreddit as well, but need help. I am terrified. I have raised my daughter who is now 5 years old since she was less than two and my wife (her bio mother) and I are finalizing the adoption. We never pushed her to call me dad, she started doing it around 2 years old. Bio dad has never been in her life. We want to tell her now as we have another baby on the way, and also before she is old and potentially holds resentment for us not being honest with her. I am so scared of how it could affect my relationship with her and breaks my heart that she may feel "different" than her new sibling. I am in tears while typing this because she is my daughter and always has been. I have gone over and above to make sure she never felt like she was not my daughter. Volunteer at school, daddy daughter dances, never miss an activity or event. I love her and my wife dearly. They are my everything. I need advice on how to handle this and could really use some encouragement. I am very close with my daughter and I have done a lot to build trust with her. I am so scared that this will ruin everything I have worked towards, or affect our relationship. Has anyone dealt with a similar situation? Most kids books seem to be geared more towards normal adoptions and not this type of thing. Please help.