you are understandably emotional @WickedMotherofthebride
Your husband loves his daughter, as much, as you love yours.
Many previous posters have highlighted the fact that his daughter didn't have the benefit of his lived-in presence throughout her life. That must have an impact.
You have had a knee jerk reaction, whilst understandable, emotionally, does not warrant you threatening divorce.
Your family dynamic is such that your daughter lost her Dad at 15, your step-daughter still has her Dad.
I suspect that if your step-daughter had already married, there would be no issue.
Personally, it would be far more powerful for you to walk your daughter down the aisle as her only surviving biological parent.
What does your daughter think?
Last thought, please look at the post from @Dracarys1 Today 14:51.
Dry your tears, don't blame your husband, you have better options tbf, it will be a beautiful day, don't spoil it for your girl.