I think this is going to be a wonderful gift for all involved. It doesn't matter if haven't officially given birth to your (step)daughter. You're her parent. She's lucky to have more than one mom. And the baby will soooo love getting cuddles from you and your husband. Sometimes I think this world gets way too hooked up on titles.
We have a daughter who's birth mother died of leukemia when she just turned 15. She was in the same alternative school as my oldest son and she was living with the mother's boyfriend who was a truck driver and never home. Long story short, she needed a mom and dad and we were it. Nothing official through the courts, but let me tell you, she has been the greatest gift we could have ever received. She is married to a wonderful man and they have 3 beautiful children. MY (AND I EMPHASIS THIS) grandchildren are the best in the world.
Enjoy everyday with the little one and just know that you're giving the baby a huge jump start on a successful life. Your husband and you have lived through enough to steer the baby through the bumps in life. And your daughter will breathe easier at work knowing baby is safe. Congratulations :):) phew... I'm wordy...