When humanoids wield an arquebus, their left hand isn't really touching it. The issue is very similar to the one occurring with a pollaxe (link).
SAVED GAME: link
Load the attached saved game.
Observe that Grieving Mother is equipped with an arquebus.
Enter the door in front of the party.
Skip through the dialog until combat ensues and everyone enters their combat stance.
Zoom in and observe that Grieving Mother's left hand isn't really touching the arquebus; rather, her whole arm is above it.