Erica had gambled everything on this encounter, and now she was worried she might have over-stepped. She might have even come by last night and saw Emily in bed with me, deciding it best to come back this morning. She might have even woken up earlier than usual—she was a late sleeper—and waited for Emily to leave. We had fooled around a bunch already but taking the literal plunge into fucking a sibling was still daunting, even to me.

"Yeah," I said. "Everything's great."

Erica gave me a small—quite nervous-looking—smile, a smile that made my heart race and my dick pulse.

Her smile turned into a sharp gasp as I...