Blood
Wendell always knew when it was coming. He could smell it on her, emanating off her pores, a musky, vivid scent that raised the hairs on his arms and left his body tingling all over with hunger, lust, and sharp anticipation. He could see it in the subtle changing of her walk and gestures, how she moved slower, more languidly, as though every movement was just a little more sensual than usual. As the time drew closer, she was sharper in tongue and shorter in patience, higher in libido and lower in her tolerance for annoyances, and her tendency to be impulsive and violent, even when unnecessary, increased dramatically.
Yes, his twin sister, Darlene was always sexy to him, but for one week out of the month, she was completely irresistible. And Wendell didn't bother to try doing so.
They didn't ever worry about locking the door. Their siblings always knew to steer clear of them when the first week of the month rolled around, and with the volume of noise they generally made from behind it, no one would be foolish enough to forget and intrude on them when the bedroom door was closed.
There were never any words; there was no need for them, once it started, and both were generally so breathless within a few minutes of their bodies hitting the sheets- or the wall, or the floor, or the bathtub- that they couldn't have formed words if they tried. The door had barely closed before they were on each other, tearing off clothing and casting them, ripped and stained and unwearable for the future, to a pile in the corner, eagerly pressing into each other with grasping hands, loud moans, sharp inhalations of breath, and occasional drawn out groans and grunts of sheer pleasure.
Darlene's nails dug deep, bloody furrows into the skin of Wendell's back, and her fangs slid downward, her eyes a deep red as she bit just below his pulse point, small bites that drew minimal blood, but were nevertheless deeply erotic for her to experience. Wendell, similarly aroused, kissed his way down her throat and sternum, between her breasts and down her stomach, scraping his fingernails down her sides and over her thighs before lightly stroking his inner thighs. Gasping, grinning in a manner that seemed almost manic with his pleasure, he removed his fingers briefly to take them to his mouth, sucking the substance lightly smeared across their tips, and then lowered his head, the better to fully enjoy. For her part, Darlene simply grinned, propping herself back on her elbows and spreading her legs wider to allow him full access as her eyes rolled back to show their whites, and she arched her throat, her hair spilling back over the sheets as she moaned low in her throat, her skin beginning to flush from her face downward before giving a prolonged shudder of enjoyment.
One thing about that week of the month, it certainly gave them satisfaction…and quite a lot of creativity.