Her sister, of course

Harley struggled furiously with the window, trying to pry open the jammed lock. It didn't help that she could barley see through her tears, her heart aching at having to abandon her sister to those demons. 'I'm okay,' she told herself. 'I can get away and tell someone. If this is a disease of some kind, there must be someone who can reverse it. I just have to get out of here and I can save her. She'll be my sister again.' She pushed harder on the jammed metal lock, finally getting it to budge. She pushed harder, nudging the latch a little more each time. She was about to give it the last shove it would need when someone wrapped their arms around her from behind.

"Please," she begged, her voice trembling and new tears appearing in her eyes. "Please let me go."

"Oh, sweetie, I already did," a terrifyingly familiar voice said in her ear. It was different, much more seductive and husky than Harley had ever heard, but she could still recognize her sister's voice. "I let you go. I told you to run. And you did! I'm so happy you still listen to your sister. Don't worry, I'll still let you go. I just need your help with something first." Harley felt two plush globes press against her back, but was more concerned with the poke she felt on her butt. She knew what that feeling was. She quickly turned and pushed her sister away, sending the purple demon stumbling backwards and giving Harley a second to take in Yara's new body. Her boobs were bigger and perkier than she had ever seen and her silhouette cut an image any woman would want. And between her legs, pointing straight at her, was a huge, meaty, throbbing cock.

"Like the new me, Harley? I was always so jealous of you, you always had the better bod. Looks like I win now though. No normal woman can compete with these," Yara said, squeezing the teardrop flesh hanging on her chest and creating a deep crevice of cleavage. "And look at this tush. Mmm, makes me want to rub my dick on it. I'm super sensitive right now, I bet I could cum just touching it."

"Y-you said you w-wanted help with something," Harley said, voice quivering, her emotions in disarray. "I'm not fucking you. I'm not letting that thing anywhere near me. And you wouldn't **** your sister, would you?" The question lingered in the air, the question now far from rhetorical. Yara's purple eyes burned into her sister's blue, the same color hers used to be.

"Don't worry baby, I'm not going to attack you. I may be an oversexed slut dreaming about having a cock down my throat, but I still love you. You're still my little sister. You can go, I'll even open up that window for you. I just wanted to give you a kiss goodbye first. You didn't give me one before."

"That's it?" Harley asked, apprehensive. "Just a kiss, and then you'll let me go? You're not going to...attack me or something?"

"Of course not, sis. Just a kiss. You're free to do what you want after that," Yara said. Harley didn't think she was lying. Besides, Yara had always been a terrible liar. She cautiously stepped forward.

"Just one, then I'm gone," she said, approaching her demonized sister from the side. "And I'm doing it from this side, I'm not getting anywhere near that thing," she said, glancing at her sister's still erect mammoth cock. Yara just smiled and reached out her hand to cup Harley's face.

"Don't worry, Harley. I don't want to hurt you. I love you," the demonic woman said, bringing her sister's face close to her own. "Now give me a kiss." Harley gulped and slowly pressed her lips against her sister's plump, purple ones.