[This was a custom request] The Halloween party you hosted has just finished and you sit alone with your older sister, discussing the evening. She's scantily clad which makes you feel slightly uncomfortable, but it's Halloween and she's dressed up like a Demon so it makes it kind of ok. Besides, you've both had a few beverages through the night and you're almost past caring. Suddenly, your conversation is halted when the lights flicker red and your sister spasms in front of you. You check if she's ok but are shocked to hear that her voice sounds completely different - terrifyingly so! She's acting weird and evil... the voice explains that it's a Demon that has taken over your silly sister's body because she dressed like one of it's own. The demon begins to taunt you by showing off your sister's body and telling you that she's a slut. It knows your darkest desires - that actually you have thought about what it would be like to fuck her deep down inside your core. You're horrified that the demon could know such things... but it's right. When it tells you that you could fuck her without her ever knowing about it, you're almost excited to have the chance. Your possessed sister lays on her back with her pussy spread and you hesitantly slide inside her. The demon laughs and taunts you for acting so despicably - fucking your own sister! That's when your sister shudders back to consciousness as you smash into her womb. She's distressed and crying to see that her own brother has her pinned down and is using her for pleasure. You're about the stop but the demon comes back and encourages you again, making your cock harder. Your sister switches back and forth between herself and the demon for a while, making you battle with your decision to keep fucking her until eventually she is back to herself and completely mind broken. Horrified at what you've done, you finally pull your cock out and begin to soften. The Demon calls you weak for giving up and teases you for your morality of wanting to stop fucking your innocent sister. Then it makes the most evil deal. You can continue fucking your sister and she'll NEVER remember this night again but you MUST reach your orgasm and cum inside her fertile womb. Then your sister will return to normal and never know anything had even happened. Your sister is mind broken so you feel you have no choice... plus you do want the chance to fuck her because you've never have it again. You nervously agree. The Demon laughs as soon as you stick your cock back inside your sister's cunt and leaves her in charge of the vessel. She's so upset and begs you to stop, but you grip her hips and plough into her. She tells you the demon has gone but you know it's still there underneath, watching while you commit this abomination. You softly explain the deal to your sister, comforting her the best you can while concentrating on the wet grip around your cock. You tell her about cumming inside her and she begs you not to - explaining that she's not on the pill and you could very well get her pregnant - but you have no choice. You couldn't pull your cock out now even if you tried with the demon's magic powers pulling you in. You fuck harder and harder, ignoring her pleas until your balls twitching out thick ropes of cum which paint your own sister's fertile cervix. You're finally able to slip out of her and the lights flicker red once more. As promised, the demon leaves your sister's body and she's sat upright confused as to why an hour has passed for her in the blink of an eye. She reaches down between her thighs to notice her oozing pussy and looks at you confused. She asks if she lost any time during the party at all tonight but you assure her she didn't and that she was always safe. As a little brother, you have to look after your big sis, right?