http://www.mangago.me/read-manga/ten_count/mf/328369/Vol4_Ch21/pg-20/
the father is saying it himself she is just some neighbour girl I think
oops sorry I misread your post I didnt see you were talking about Kurose lol, I thought you were talking about shirotani... oh well she is not kurose sister as well because he is a only child ^^
http://www.mangago.me/read-manga/ten_count/mf/328369/Vol4_Ch21/pg-29/

Ok so they're not brothers....but what if she's kurose's SISTER??!?!!