Saying “my stepmom” instead of “my dad’s wife” is a small word change with a lot of welcome in it.

One word can do a lot of quiet work. The longer phrase holds somebody at a formal distance and has to be restated every single time, while the shorter one simply puts them in your family, which is where they already are if things have gone well. Nothing is being faked, and the small change tells the room what to assume.

Use the word that matches how it actually feels, and let that develop at its own pace. If it is not there yet, do not perform it, and if it is, stop explaining the arrangement to people who never asked. She will notice which one you chose.