Hey! Back again. :)
There were a lot of little details in this that I really liked and that made the story rich and vibrant to me. I loved the mentions of her family members in the beginning - you worked them in in a way that felt realistic and natural, and through a short mention of the last time they were all at the Burrow you gave me a good sense of Molly's place in her extended family. Later, when she's at dinner, I also felt like her thoughts about James and Fred as well as her conversation with Louis helped expand on what was already mentioned in the library.
Your portrayal of Molly from the start was also quite strong. You didn't make the narrative so overpowering that I felt like you were hitting me over the head with it, but there were a lot of subtle little comments that went a long way. Little things like how she has practise and discipline in abundance, how she'd prefer to be anonymous sometimes, how she's not interested in the family meetings (which, incidentally, are a neat idea), how she tries not to read when she walks because she tends to walk into people... you've really painted a thorough picture of who Molly is as a person.
I also like the fact that you haven't just leapt headfirst into the plot. I like the fact that you have taken the time to just develop Molly as a character, especially since from the prologue it seems like it's going to be a fairly character-driven plot.
There was really only one issue I saw in the chapter.
While I liked second person in the prologue, the fact that you seem to be sticking with it for the rest of the story gives me pause. In some ways, I do like it - it keeps a removed, eerie kind of feeling that I think works well with Molly's character and with the subject matter - but in others, I think that it's a bit awkward. Some of your descriptions in particular don't really flow naturally (especially around the scene in the Great Hall), and I think it's in part because they're being told in second person.
I'm going to assume that you chose second person for a reason, especially since, as I said, I think it probably will work well with the overarching plot, so maybe there's a way you can rephrase your descriptions to make them work better for the pov you're using. As it is, I feel like you're just writing the way one normally would, and just switched "she" or "I" to "you." Because "you" is a different perspective, I don't think it really works as well as it could.
What would someone talking to Molly - who is fixated on Molly - be paying attention to? What would they be saying? As you talk about Louis - saying that his voice told her he was grinning, that he rolled his eyes, etc - why does this person care? Personalise it. You're doing a great job of characterising Molly, but I think the narrator should have some sort of character, too. Otherwise, what's the point?
That aside, though, this was another great chapter, and I really enjoyed it. :)