Well, it's short enough so far it's hard to comment on the characterization. Oddly though, from what I can tell, your characterization problems aren't with the OCs so much as the canon characters. Hermione's not acting so much like Hermione grown up as she's acting like Molly. Also the pacing's a bit rushed in spots...try to even it out if you can. I'd be more specific, but my brains a tad fuzzy just now.
My favorite part of the story is the tone of the writing. The POV shifts slightly, but it's an interesting one. I like the way Hermione and Ron are refered to as Mum and Dad especially. It's not something you usually see and is really quite effective.
For future chapters - more physical description. We have all of these names, but no faces to put with them outside of the canon characters.
Do keep at this, please. I'm fond of OCs and it's refreshing to see someone out there aware of the MS trap and trying to avoid it.