Or if you prefer the English title: Lately, my little sister's been a little weird.
A RomCom anime. A boy suddenly gains a step-little sister and said sister gets possessed by some ghost/spirit that needs to fall in love with the brother in order to "move on" to the next stage.
The art isn't too brilliant.
Apparently step-brother-sister isn't taboo in Japan.

This is not my cup of tea so I'm dropping it. But if hearing a young girl calling you "onii-chan" tickles your fancy then by all means follow this series.