Auntie is invisible on the small wooden bridge to the far left of the entrance. To find her, just take any AoE targeting (Bane, Alchemist Fire, etc) and put it on the bridge. You'll see the red enemy circle. You can 'attack' her this way to start combat, though she might bug out and just stand there.
My guess, she was meant to be 'invisible' and ambush the player as they rushed to save Mayrina. Just the code isn't working properly so she stays invisible.
