I went through a similar thing with my DD who asked a lot of questions like that, she also cried when she figured out we'd had a miscarriage (at 4 mind you! She's a smart cookie!) and I had to explain to her that sometimes a body can make mistakes, or get confused and not make a baby properly or not even make one at all when it was supposed to. And that it would hopefully happen it just might take a little while. I didn't compare myself to others so that she wouldn't think there was something wrong with me (thats my burden and I didn't want to share it with her @ 4). She took it very well, and eventually we were lucky enough to have another baby but she did understand that it might not have happened. Like your daughter she would continue on with discussions about her sibling as though I had never spoke, but I took that as positive thinking and allowed her to do that.

I think they are stronger than we give them credit for, its us who has to deal with our own grief, desperation, anger and then the concern for them on top of everything else.

You are a wonderful mother, and I have everything crossed for you that your time will be soon