What is Kurumi's Gender?
Kurumi is a girl, she told the main character that she used to be a boy, but it turned out she was just messing with him.

We can be fairly certain that there was no more to the joke as whilst the main character was stressing out and trying to judge his feelings on the matter, Kurumi had forgotten all about it by the time he approached her again about it.

The claim about sharing a father was similarly false