Rei chan is the step brother of Yuki's father. Which is Houdou san's bff
Half brother. The boss sanada was father to both ryuuji and reiji (and hisako) but the mothers were different. I think ryuuji's mother died and reijis mother was a second wife but I'm not sure
Isn't step brother and half brother are the same? But yeah, that's what I meant.
Stepiblings are the children of the parents' new partners (so the new parten comes into the household with his/her own kids) whereas half siblings are the parents' children with someone else. So basically one is connected via a new marriage and the other is genetically related.
I really cant remember story but all i know is rei and yuki have special relationship so maybe this is the best way to safe yuki and his lil bro?