Corrupt her

An ample smile formed in your face, looking at your usually pushy sister now begging for you.

"Oh, I think you need to better than that. For starters, you will only call me Master, from now on"

You could see the conflict in her face, at having to lower herself to that level. But the look of your erect cock, throbbing just inches away from her pussy, took away her doubts.

"Yes, of course, Master! Please, fuck me now!"

You got closer to her, removing the pitiful toy from her and teasing her entrance with the tip of your hard penis.

"One more thing. You can have this, you can have eternal pleasure and beauty... if you give me your soul"

She didn't hesitate this time. "Yes! Yes, you can have it! Now please, fuck me!"

You didn't need more. You pushed your cock into her, making her cry out in pure bliss as her soul darkened and then spread the corruption to the rest of her body. You fucked her roughly as her breasts expanded to DD cups and her ass ballooned outwards, while her waist thinned, giving her a full hourglass figure.

"More! Give me more, master, please!", she begged. Your cock was pistoning in and out of her, speeding her transformation. Horns came out of her forehead, and her skin took a scarlet hue. Her eyes were deep pools or black, with slitted eyes in the center. Her perfect body was pushing you towards the edge.

"Enjoy your new life, slut!", You said, releasing your cum out of your heavy balls. Her demonic body took most of it, but some spilled outwards, the black tar contrasting with the white sheets

You pulled out, enjoying the sight of your first corruption. You felt stronger now, eager to take the rest of your family at once.

"Come on, sis", you said, both of you turning into improved versions of your old self. "We have work to do"