On her back

Matt rolled her onto her back, moving over her as he did. The hand not supporting him over her moved down her body to pull her shirt up and over her breasts, exposing their soft curves and delicate nipples to his gaze. He roughly grabbed one, then the other before moving his hand down her smooth stomach to pull the fabric of her skirt up to expose her shaved pussy.

Ignoring the helpless look on her face, he unzipped and unbuttoned his pants, moving it down just far enough to let his hard cock spring free. Holding it, he rubbed the head up and down her slit, still slick from their dad, before pushing the tip inside. Lily hissed as he pressed down, his cock spreading her labia before penetrating deeper as he thrust down hard. Matt gasped at the tight feel of his sister's pussy clenching around his cock, ignoring the sob that escaped her lips.

The hand that had guided his cock into his sister moved up again, clenching her breast as Matt thrust again and again, fucking her hard in her own bed. Unfortunately, his inexperience and excitement pushed him over the edge much faster than he had wanted, and before long, he felt his orgasm crash over him. He thrust in harder, pushing his cock as deep inside Lily as he could before cumming, adding to their dad's sperm inside his sister.

Matt trembled over Lily, holding still until he finished cumming, then collapsed on her. He could feel her breathing hard underneath him as he breathed 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."