Sibling Birth announcement ideas?

Kerenza
I'm way ahead of myself here as we've agreed not to announce until after the 12 week scan to the girls but I want to start thinking about a fun way we can announce the birth to my boyfriends girls. 
It's my first pregancy but he has two beautiful girls who are 12y &13y. Any ideas of fun ways we can tell them they're going to be big sisters?