"Hey Sara? You doing ok? Did you do fine on the tests?" Stan asked. It was a day after the exams and the two siblings were back at home.
Sara sighed and said "Yes, I think I did well... It's just my roommate was a succubus and... She said or thought I was a succubus as well."
Stan chuckled "Hey we're brother and sister right? How could you be a succubus?"
Sara nodded and said "Yeah, we're brother and sister." It didn't quite help Sara though. It may have been her chest swelling a bit quicker than usual, or some odd senses... There was something about Stan that made him more... interesting, and their father had a lot more of that something. It was getting late and Sara decided to take a shower. As the water came down and Sara started lathering she felt something on her head... They felt like hard bumps... Were they horns growing in? Scared she checked her butt around her tail bone, and felt something down there... Maybe a tail starting to form? Sara sat in the shower for a bit as she was trying to process what was going on. The fact that yes she was a succubus and her body was starting to corrupt into a demon of lust. She didn't feel like having sex with everyone, would her change also corrupt her mind and then would she really be Sara?
Getting out of the shower and drying off Sara went to her mother who was at a table reading a book. Stan was off in his and Sara's room, and their father was off at work letting Sara ask "Hey mom, Can I ask you something important?"
Her mother said "Sure, what is it?"
Sara asked "I'm not actually Sara am I? Not Stan's twin?" her mother was silent thinking about what to say as Sara said "I've got horn stubs on my head, and a tail starting to grow. If my roommate during the tests was right I'm a Succubus not human."
The lady who raised Sara said "Yes... Something happened to that first Sara and we tried to cover it up a little."
"Were you ever going to tell us?"
"Depending on how things went we figured maybe we wouldn't have to... Considering-" Sara's mother brushed her hair and felt the bumps too and continued "your changes. I think we'll have to tell you both... or at least just Stan at this point." The two were silent for a bit.
Sara trying to keep things a little more lighthearted said "Well... Ever bet I'd be the one to be some sex crazed demon out of the two kids you were raising."
Sara's mother went to the kitchen and poured herself a glass of wine and took a long drink then said "There's a chance Stan still could be the sex crazed demon out of the two of you." Slight chuckles among the two girls. The family had been preparing for the twins to be separated and that night moved living conditions around just a little. Now mother and daughter would sleep together and father and son would sleep together. That said Sara would quickly become more and more restless in her sleep.