He gives in regardless and pushes back in

He stares, transfixed, chest heaving. The sight of it—his thick, white load streaked across your perfect ass and dripping over your open, glistening pussy—is too much. The last shreds of restraint snap. A low, animal growl tears from his throat as the overwhelming urge takes over.

Before the final spurts even finish, he pushes back in—hard. The head of his cock, still pulsing, slides through the mess he just made and thrusts straight back into your slick heat, dragging his own fresh cum with it. The sensation is filthy and perfect: your pussy welcoming him again, now even wetter, warmer, lubricated by his own release. He buries himself to the hilt in one rough stroke, the last weak jets of his orgasm spilling directly inside you as he grinds deep, hips locked flush against your ass.

Fuck… too late, he thinks dimly, pleasure and panic crashing together, but he doesn’t pull out again. He stays there, buried inside, feeling the warmth of his load coating your walls, seeping deeper with every involuntary twitch of his spent cock.

After a long, shuddering moment he finally withdraws, watching a thick river of cum—now undeniably inside and out—slowly leak from your stretched entrance. Guilt floods him, but it’s drowned out by the darker satisfaction thrumming in his veins. He knows, deep down, that some of it—enough of it—is already exactly where it needed to be.

The line has been crossed.

Your brother comes down from his orgasm and realizes what he has done. Hastily he puts your G-string back in place and throws your blanket over your body - hoping to conceal his incestuous deed. Silently he vanishes from your room and sleeps on the couch...