Sarah is confused

Sarah had never seen such a huge cock in her life.
Only about a month ago she had a threesome with her best friend and lover Amy and Amy's cousin Carl. Carl had an impressive 9 inch cock (they had measured it), but the memory paled in comparison to the sight of her brother stroking his massive cock. It was definitely longer, much thicker, veins producing on its surface and glistening with precum.

How could her brother possess such an intimidating monster? Without her ever noticing?

"Adam? What? How? What is happening?" It was becoming more difficult for Sarah to concentrate and keep a clear thought.