Oops... I Came Inside My Step Sister! She Asks to Stay the Night in Her Step Brother's Bed

The neighbors next door are too loud, so she knocks on her step brother's door in a short satin nightie and asks for one quiet corner of his bed. He shifts to the wall and gives her the edge, the room goes dark, and for a while nothing happens at all. Then a hand slides under the blanket, finds him through his shorts, and neither of them says a word about it. What starts as a favor turns into a slow, breathless night under one blanket: her palm, his hand on her hip, the two of them pressed together on their sides in the quiet. She stops him once — this feels wrong — then pulls the nightie over her head and keeps going anyway. It ends the way neither planned: one thrust too late, wide eyes in the dark, and a whispered "...it was an accident." Intimate POV, real bedroom, no script for what happens after.