This page of the Baldur's Gate 3 guide has a walkthrough for a Deal With Auntie Ethel quest. This walkthrough will help you complete the quest successfully.
- Get Auntie Ethel's help.
Heading towards the Riverside Teahouse, you will find two brothers - Johl and Demir - accusing Auntie Ethel of kidnapping their sister (M7;1). If you pass the investigation skillcheck, it'll turn out the woman is lying. During this dialogue, you can take the side of brothers and help them in their search. After the talk, the woman will disappears.
To find her, you have to go deep into the swamp. If you pass the secret knowledge or persuasion skillcheck, it'll turn out that the whole thing is just an illusion. If you fail to pass it, you can also take down the illusion by talking to the sheep. Failing will result in the neighborhood still looking incredibly colorful, as in the picture below.
Auntie Ethel lives in a Teahouse (M7;5) located near the point of fast travel (M7;2). As soon as you go inside the house and decide to talk to the old lady, the cutscene will start.
During it, the witch will offer you to remove the parasite in exchange for one eye. If you agree, it'll have practically no effect. All you will find out is that the Illithids inside heads of your team members have been altered by magic. Additionally, you'll get a permanent status, which worsens the character's perception and prevents them from performing a critical hit.