Oh Julia, I can already tell that I'm going to love this due to the description you have in the first paragraph. Love love love, and classic Julia. :)
Wait. This is straight up in third person. Holy crap on a stick.
Ok so I have just finished and I absolutely adored it. Maybe not the kind of warm and fuzzy fare that one might expect on Christmas eve, but it was a great story! So heart wrenching and painful.
First of all, I don't know how you can possibly say that you can't write action because this had a fair bit of action in it and it was masterfully done! The scene where you describe the werewolf attack is especially frightening, and I think the fact that it was done from Eliza's telling was great. She is a very interesting character, completely haunted and torn apart by what happened to her sister.
And while her sister seemed to live with it, to cope with what had happened to her, Eliza seems to fall into a weird PTSD kind of stupor that only continued to spiral further out of control.
But let me back up a bit because I think it's really interesting the way you've started this out. It's more of a quarrel, two sisters fighting a bit, already showing off this animosity which, despite the fact that they may have been friends other times, really kind of sets the tone for this story, that it might not have a happy ending.
And then, when they are at Hogwarts and Ellie has friends and such, and Eliza spins deeper into madness, I think you were excellent at capturing that descent into madness of Eliza's. I really also liked the way you mentioned that this was next gen, the way you casually were like, oh, potter and weasley clans, but you didn't dwell on it, you moved on. I really enjoyed that because it was a clever way to kind of tie the series to canon, rather than just a story with straight, unrelated OC's.
Finally that last bit, with the fall. How chilling you wrote it, with Ellie trying to reason like any normal human being would, and Eliza just completely past reason, everything so out of focus for her. I like how ambiguous the ending is too (despite the fact that it kind of HAD to be ambiguous as guided by the TOS). Part of me hopes that something ended up saving Ellie. You know, sometimes wizards can save themselves with bursts of magic and such, and even if that's not what you intended, that's what I've decided in my head, because it's Christmas and it's happier that way. Yes, so Ellie lives, and Eliza...well, she probably doesn't but whatevs.
Ah Julia this is such a great piece here, and so under reviewed. I wish I could just take your entire author page and put it on story seekers because you're such a brilliant author, you have an absolutely superb way of getting your characters across and making the reader empathize/sympathize or whatever. Even your third person is fabulous, and it absolutely is not bad at all (even though you always say that it is. It is NOT so shut up. :P ) You understand stand your characters so well, and it shows. You have such vivid descriptions and vibrant word choices it's just great. I loved it.
Merry Christmas Julia!