Op I think you are not quite understanding something, and I know it's hard to listen when you're feeling attacked. 
It sounds like you have done a great job at preparing her for the new baby, and working with all the family to ensure joined up stance, saying the right things, trying the right things etc. it sounds like you're doing a really great job :)
The problem is that just because you are doing EVERYTHING RIGHT, doesn't mean to say your sdd is going to 'get' all the right messages and react right either... It's a sad fact of parenting, that we can only do so much and cannot control what's going on in their heads or hearts.
I wonder if she is still ashamed, or down on herself about it, or wants to be private about it, and hide it... Just cos you tell her not to be, doesn't mean she'll even know where to start trying not to feel her feelings IYSWIM? The amount of times I have to say simple things to my Ds, is ridiculous! And he's 4 so at a nice mouldable type of age... By the time they're eight, they are so much more their own person and get effected by a huge range of influences, so it's hard to get a message through to her and actually have her probably believing it...
Im wondering what would happen if you just nudged yours / dads etc behaviour just a tiny bit and behave as if you think she might be embarassed and worries... So, just do what you do but maybe repeating yourself about don't be embarrassed once more than usual, stuff like that?
It's not a reflection on you at all, it's just about trying a few different ways all with the same idea (don't be embarrassed well done for telling), just to try and get the message sticking on her brain a bit more - I suspect it's her brain stopping her from hearing you all!
What do you think?