Sam woke up fast. "I'm intrigued, what do you have in mind?"
She didn't answer, and instead took his hand and led him out of his room and into the shower. Without a word, she turned on the water, slipped out of the robe, and guided him inside. Once they were under the water, she leaned into him and wrapped her arms around him, holding him close. He put his arms around her and stroked his hands up and down her back. They stood their holding each other in the warm water for a long time.
After a while she reached over and got the massage oil, and poured a ridiculous amount of oil in between their bodies. Then she slid her body up and down against Sam's spreading the slick oil in between them. His hard cock quickly became covered in oil, and it felt amazing sliding across her oily body.
Then she turned away from him and put her hands up on the wall, bending forward slightly and inviting him to rub his cock on her ass. He stepped forward and put his hands on her hips, lining his hard cock along her ass crack and sliding it up and down. He poured more oil on her back and watched it slide down onto her ass and his cock. He watched mesmerized through the steam and splashing water of the shower as his cock glided through the oil and spread it around on her ass.
They had put so much oil on that the contact between them had become super slick, and as he slid his cock downwards, it suddenly slipped forward between her legs. She froze for just a moment as they stood there in the shower, his hard, slick cock poised at the entrance of her warm, wet, oily pussy.
Neither moved an inch, not up or down or forward or backwards. They both just stood there quietly in the shower, feeling the intensity of the moment. The head of his cock was right there at the forbidden zone, the place where they had both said they didn't want to go. But Sam knew he couldn't stop now, and he could sense that his sister wasn't stopping either.
Still, neither moved a muscle. Even though neither of them were moving, it felt like his cock was very gradually moving up inside her. Gradually, almost imperceptibly, his oily cock inched upwards, and her oily pussy inched downwards. After a good five minutes had passed, Sam felt like the head of his cock had fully entered her. Another five minutes went by, and he felt like another inch had eased it's way inside. Then it felt like he was halfway inside, and they both realized it was inevitable.