How to tell my step daughter 🤷‍♀️

So I’m 17 weeks tomorrow and we have not told my fiancé’s daughter yet due to a previous loss. I’ve been telling him since 12 weeks I want to tell her but he’s been hesitant. She has been his whole world for almost 10 years she is use to all of his attention as he’s spoils her. When I first came into the picture 4 years ago it was quite difficult but now we are super close and she was happy to find out me and daddy are getting married. She has 2 younger brothers at her mums who she constantly moans about so I know she loves coming here for all of the attention.

I’m hoping as we are having a little girl she will be happy as this will be her first and only sister and she said she wanted one (she wanted her mum to have one( she has never asked if we are going to have another baby.

How do we tell her without her getting upset? She will never ever be pushed out but I don’t want her thinking she will be. She’s very sensitive so I want to do it the best way for her.