(EDIT: Here's a copy of the file, the only difference is that I had to remove the cloth simulation to fit the file size limit.)
I'm new to Blender, and I've been struggling with this for a while. Everything else seems to be working fine, but whenever I rotate the bone joints to reposition the arms or the wings, a large area of the wing mesh gets pulled toward the arms. I don't know what I did wrong or how I can fix it. I originally parented with automatic weights, then tried with envelope weights, then tried with empty groups. I've tried with and without weight painting, which I honestly don't understand in the slightest.
Setup notes: the bones for the wings are in a separate group from the rest of the armature. The model itself is divided into separate objects; hair, body, sweater, and wings. The sweater has cloth simulation on it, though I'm not sure how that would affect it. (side note: if anyone knows how I can make just part of the sleeves and bottom of the sweater loose and floppy without changing the rest, in a way that would work with a playable character in Unity, let me know please!)
CONTEXT: The character's wings should be able to flap, and this character is intended to be playable in a Unity game.