Imouto no Tomodachi to Himitsu no Kankei

Saeki Sana, my younger sister’s friend, often comes over to our house to play.
She’s a very shy girl, but we share a love for anime and manga.
After talking a few times, we became friends.
My sister isn’t interested, but we decided to go see a movie of a series we both like.
After the movie, Sana confessed her feelings to me.
I was so happy that I immediately said I’d go out with her, but…
My sister had warned me beforehand not to make a move on Sana.
What would our parents think about me dating Sana, who’s much younger than me…?
“Um, would it be okay if I dated you, big brother, without telling Hikari or our parents?”
And so, our secret relationship began.