It's Stephanie, his youngest sister.

John could tell from the vague outline and the singing that it was his eighteen year old sister, Stephanie. She had short, dark brown hair and a very pretty, shining face. She was still growing into her teenaged body, but John had found himself ogling her more than once when she came home wearing a bikini top from tanning with her friends or with Kelly.

John was frozen in place for a few moments, watching his youngest sister clean herself beneath the warm rain of the shower head.