On her stomach

At first, Lily resisted, but not for long. Apparently their dad had moved her enough for her to realize that struggling would only delay the inevitable, and she rolled onto her stomach. Matt rolled with her, laying on top of her as his knees pushed her legs apart, then reached between them to unzip and unbutton his pants to pull his hard cock out. Putting one hand on the pillow next to her face, Matt moved his hips until his cock was pressing against his sister's pussy. He thrust down hard, reveling in the feeling of his cock pushing into Lily's tight pussy. He groaned and she sobbed as his pubic hair pressed against the fabric of her skirt that lay between them. He moved his hand up, flipping the fabric up so he could feel her naked ass against his pubis, and propped himself up so he could watch the side of her face as he started pumping his hips.

Inexperience and excitement led to his downfall, however, and it wasn't long before he felt his orgasm crash over him. He thrust in as hard as he could, holding tight against her ass as his cock spewed sperm deep inside his twin sister, who just lay there sobbing. Finally he collapsed on top of her, breathing hard into her ear. "If I knew how good that was, I'd have joined dad on our birthday," he gasped, feeling her shuddering underneath him. "I guess that pussy of yours is going to get quite a workout from now on."