Congratulations!

As you seem to have thought of already - bear in mind that this is your big joy, not hers and she may not be thrilled, she may like your bf but still not want him to play a larger role, she may feel sad or angry that her dad is now definitely, irrevocably out of the picture, and she may feel left out herself. Feelings are funny - tell her like an adult, but accept that she may react like a child, at least inside.

I didn't like my father's second wife for the longest time because I felt she took my dad away from me and I could never talk to him without her being there and butting in. And they married ten years ago or so! We get along fine now, though.