Ah, the news from hell! I think you did a wonderful job with this story. Emily's tension comes through beautifully. Your touch is light, and yet it's has a realistic feel.
The only thing I'd suggest, in terms of CC, is that the Hogwarts letter really comes out of nowhere after you've set it up with father and daughter being muggles and dad, against all odds, marrying a witch. After that, when daughter turns out to be a witch, I kind of went "huh?" The reasons are that (1) it's too great a coincidence after dad marries a witch and (2) she'd have shown some sign of it before she got her letter, which stepmom would surely have recognized even if dad didn't. There are ways around it -- she may have been experiencing magic but been too confused to tell her dad and too scared not to keep it from stepmom (who would surely have questioned her when she got the letter).
I don't want to sound like I'm being critical, because it's just a small thing. I think you did a really good job with this story.