Deal with Yuna.

Before Yuna can say anything more, you grab hold of her neck and slam her up against the wall.

"Well, Neir, it seems like my dear old sister is a peeping tom..." You say.
Yuna's hands come up and try to pry your fingers from her neck. However, your grip is much too strong.
"That she is, mistress... That she is..."
"So, Yuna, how much did you see? You only see the pop? Or did you watch the whole ride there?"
Yuna chokes out some half-formed syllables.
"Miss, you should ease up, you're likely to kill the poor lass!"
"Oh, fine."
You let go and Yuna drops to the ground. She sucks in rough mouthfuls of air whilst trying to crawl away from you.
"Not so fast!"
You pounce on her, holding her down with all of your weight. She looks up at you with terror in her eyes.
"Wha- What in the hells are you? What did you do with my sister?" She says.
You can't help but laugh.
"HA! You're a bright one, aren't you?!"
Neir comes over and squats down right above Yuna's head. Still dripping wet from Jace, Neir rubs her clit with a pair of fingers. A thick drop of clear liquid drips from between her lips, landing on Yuna's forehead. Yuna recoils at this.
"Cannae have a go at her, miss? I've been hungry for her for weeks." Neir says.
"Maybe. If your good."
"Certainly."

"So, sister, what was it that tipped you off?"
"I- ah -heard something. Heard you two. When I saw... Oh gods... Your eyes. Not Mell's eyes. Not even close."
You absent-mindedly pick at the bottom of her nightgown.
"Uh-hun..."
Her voice gets more and more manic as you lift up her clothes.
"What- what- are you?! A mimic, a changling?"
"A demon, actually." You say.
"How? How did you-"
"I took her soul. Mell's, I mean. Replaced it with my own. Best decision of my life."
You pull at a hole in her nightgown. You pull and pull until that rip splits the hole damn thing in half. Yuna's pert body is fully exposed to you. She looks quite similar to you, actually. Amber skin, perky breasts, the works. Not as fit as you are though.
"This flesh... Dear gods, Yuna, it is so damn intoxicating..."
With Neir on one end and you on the other, Yuna is trapped. You lift up one of her legs and nuzzle it, licking up the sweat off of her skin.
"Being in this house with that silver fox, oh, I just couldn't help myself..."
You press your cunt against hers. You roll your hips once and she cries out. Whether it's from pleasure or despair, you cannot tell.
"Don't worry. He won't remember a thing in the morning. Neither will you. All you'll have is a sweet, sweet fantasy of your dear baby sister."

You fuck the poor mortal girl until she cums. She starts crying afterwards. Figures.
When your done with her, you hit her with Trancen; a more powerful level of the Trance spell. You carve the memories out of her mind. The memory of you and Neir's little jaunt with Jace, the memory of your confession. You leave the memory of you fucking her though. With no context, the memory of her sister rocking her world like that should do strange things to her.

Gods, You haven't had this much fun in years.