H.B.
It is NOT going to damage her in any way to think she has a moon daddy at age 3. Most people are raised to believe a big, fat guy comes down the chimney to deliver presents and a little fairly takes your teeth. A moon daddy is no worse.
If the stepmom is an otherwise good person and treats your daughter well then be happy. Kids are very resilient and smart.
You said "she is going to make my daughter her daughter". . sorry to tell you this, but your daughter IS already her daughter by marriage whether you like it or not. TRY to have an open and honest conversation with the stepmom and your husband, or if you are more comfortable, with just your husband.
If I was the stepmom, I would fee like you were making something out of nothing. When these girls are 8,9, 10 or whatever, they will have asked more questions about family stuff and will know who is who ... for now, if it creates less drama for the two little girls and makes them feel equal in a new home/family then let it be.