Welcome to Gaia! ::

Alright, so I think that you should use the miniskirt (preferred in black) and the purple shirt. Then, as you have mentioned, totally take the shirt and write "Twisted Sisters" in "Metallica" kind of writing. Other than the hair, boots, cape, and choker, plus with you already about to get a wand and *snicker* pig snout, you're covered! ... As for your friend's Ginny Weasley cosplay, I'd go with this look shown here. She can choose between the black pants or a dark gray/black skirt (whether pleated or not is her decision); either is fine~

I'm, personally, not a cosplayer, but I am an artist, and I've done a few commissions of the Harry Potter characters. They're fun to draw, and I like being creative in helping create my friends' costumes and such to make them look closer and closer to the real things.