<Then Rachel remembered she was still naked, but before she could put anything on the door opened and her husband entered the room. It was:>
.. obviously a man, a tall one. Even if under his thick moustache, Rachel could easily see her older sister, or her former sister, now changed into a man. She/he approached to her/his wife (ex-little sister) to kiss her gently on her lips.
Rachel felt weird to be kissed from her sister, or from the man she now had become, but felt very aroused too, then kissed her husband with passion and lust.