Telling first child!

Ok so my first is 9.5 years old and I'm so scared to tell her she's gonna be a big sister.  For so long it's just been me and her.  She loves my SO, and is transitioning to calling him dad, her decision no prompting from us.  But when we even joke or bring up baby's  to test where she's at she says no she doesn't want to share us.  Any ideas how to tell her would be a great help.