They are not related despite having the same blood type and the same FINGERPRINTS even. Least spoilery answer is that they're in a murdergame thing where there's two teams with a King, Bishop, Knight, and Pawn on each side. Everyone has the same face as their counterpart piece, and Yuki & Ruka are the Pawns :|b Haruki who was just accepted to Amethyst is Kazuki's counterpart.

this has caused DIFFICULTIES in Aather since. Ruka murdered Yuki's family, tried to kill Yuki, and wanted to usurp his identity as well :Db

NOT KNOWING ABOUT RUKA AT ALL IS A POINT IN SWIFT'S FAVOR because pawns are clingy, insecure, and also hate being mistaken for each other. my friend '-' my friend only.