Itsuwari no Kyoushitsu

Having refused going along with his parents' overseas transfer, the protagonist is sent to reside with his cousin and childhood friend, Nakamura Yuuri, who also lives alone due to family circumstances.

"My cousin Yuuri... It's been ten years already since then."

The protagonist arrives at the Nakamura full of anticipation - however what welcomes him is not the bright, cheerful Yuuri of old, but a timid, downcast, albeit beautifully matured girl.

Quietly, Yuuri speaks.
"We'll be attending the same school starting tomorrow, but... Don't tell anyone we'll related."
"We probably shouldn't speak to each other at school, either."
"Otherwise... You'll get bullied as well..."

[Adapted from the official site]