There is no point in telling this girl that she is number 1 priority - words mean nothing, and she has been on the receiving end of discovering that parents have put her last.
The stepfather lavished her with attention and then just disappeared! See all the threads on here about ghosting, and grown women getting upset by men they've known a couple of months disappearing? Can you imagine how hard this has been for her?
Telling her she's a priority sounds like bullshit - because in her mind being a priority means not going on the date. I don't think it's right that she dictates that - but you have to be clear on what she means by priority before you think it's in any way reassuring to say "you're my priority".
I would postpone.
I would tell her that it is going to happen, because mummy gets to choose that - it's important she doesn't think she can stop things. But I'd tell her that as it's upsetting her so much, I have moved it to +2 weeks so we had more time to talk about her feelings.
Then I'd get a babysitter for the son and spend an entire day love bombing the girl. Lots of time together.
Then next day, I'd approach her again to talk about her fears. I wouldn't dismiss them. Like. I wouldn't say "no that's not going to happen". I'd say "I can see why that worries you - what can we do to reassure you?" or "let's make sure we talk about that to make sure it doesn't happen".
And I'd consider a play therapist or regular therapist with family experience. I don't think what this girl has been through should be underestimated.