From Tokyopop:
Through a series of coincidences, Yoshinori wound up having to fill in for his beautiful older sister as a model for a bridal gown. Now, he has discovered that he has a liking for cross-dressing, and that by doing so, he is learning a great deal about what it means to be a woman--and a man.

Contains a short story entitled "Reason" in volume 2

From FairGame:
A boy named Nishizawa Kazuya is in his second year of middle school. Unlike his younger sister, Rika, he is good at a lot of things and is the fastest runner in school. Parents and others think he is a responsible and hard working child. It seems that he is a perfect and good child but he has a secret that he can't tell anyone.
Every day he enjoys taking a peek at his sister undressing herself.