So many possibilities. Being so young, I assume it’s gotten bigger over the past few days? Did you hatch the duckling, or did she come from somewhere else? The pics look like she is in a brooder inside? Have you had her outside where she could have gotten into anything, or the bee sting possibility, etc?
I had one with a similar lump around her eye. The most likely theory I came up with was a straw sliver from some bedding that the momma duck pulled into the nest. Maybe your sister can tell if it could be drained, or if it would need to be surgically excised?