It's a tough one as I completely get how she feels, I'd feel the same. I wouldn't have even wanted my mum and dad to have another baby when I was that age, and they were together! I am worried about this too currently as I am pregnant and my SCs are 8 and 11, they don't know yet. Really hope they aren't upset, but am aware they could be.
DP is just going to tell them, alone, and then that's it - they can talk to us about it whenever they like, but I'm not going to try and 'encourage' them or involve them in any way. That's probably not helping your DD (from her SM and dad) as she'd probably rather not have to think about it every time she's with them.
The reality is that she will cope with it when the baby is here, and it's a lot easier to see things are ok when it's actually happening - but in the meantime, I think they just need to steer clear of the topic unless she brings it up, not go on about it around her, and just be normal.
As for her not going round there until she's got her head around it, I think this is a really bad idea as then she will just build it up. Best for her to continue as normal and be reassured by the usual routine. I think it's fine for her to prefer not to stay overnight when the baby arrives if she's worried it will disturb her, but I think everyone should impress on her how much she means to her dad and always will, and he still very much wants her to come as normal.
I honestly think the months in the run-up to a new baby are the hardest for the existing children, it's a massive unknown and causes a lot of new feelings. But hopefully when the actual baby is here and they get to build a relationship, as well as see that they will not be pushed out, things will be ok.