On your sister's birthday the best thing you could do is tell her to ask you whatever she wants. Thereafter you find yourself in the bath talking a shower with her.
Short Story: Kotori Birthday.
Btw. I found this yesterday. I don't want to assume but someone needs to translate this.
Fuck. Someone should buy the manga and scan it, if it could be found.
Good luck with that then considering this thing was released like 3 years ago and we never got scans because everyone else was busy with the game, short stories, LN translation and new volumes.
It would be great if someone found it but I tried my best and didn't find anything.