I think this bug begins when you decide how to talk to her and Mayrina before discovering the hidden path through the furnace. The first time I did this quest, I opted not to tell Mayrina about her brothers, therefore not making her angry at Auntie Ethel. I arrived at the bottom of the dungeon, Ethel was there but not aggressive, I killed her, freed Mayrina, looted everything and revived her undead husband. Got credit for completing the quest. The last 3 times i exposed Ethel to Mayrina by telling her about her dead brothers, and each one of those times by the time I arrive at the bottom of the dungeon Ethel is not there and Mayrina will not speak to you after releasing her. Essentially this breaks the quest chain logic preventing any option of finishing.
