S.B.
I second the family counselor idea.
When my husband and I married, his son was 11. We moved very slowly (dated for 4 years before marrying and moving in). We saw a family counselor for a short time last summer to resolve some communication issues and I wish we'd done it years ago! Even just a few sessions can be so helpful! The counselor may also be able to help her adapt to the massive changes and accept the new family members without feeling like she is replacing her father.
Good luck!