It is their prerogative, and responsibility. I'm sorry to rain on the parade, because it sounds like a such a joyful thing for you and your daughter. But it doesn't sit well with me that the adad is doing this on his own. His exwife is still the girl's mother and the girl is 15. This is a huge parenting decision and should have been decided jointly regarding their child and he should not be counseling his daughter to keep secrets from her mother, especially one so big.
I don't blame you for it and I don't blame you for being excited and joyful. But I have big issues with what the adad and stepmom are doing with regards to respect for the girl's mother and her relationship for her daughter.
For everyone's sake, I hope this ends happily. Best wishes.
