Anon, there's no such thing as "related soul wise" This relation thing only works when biology is involved which isn't in yuji sukuna's case considering the available info. Not to forget gege nd not a single chara from jjk has talked abt yuji sukuna's supposed relationship compared to choso yuji, kenjaku yuji. The only exception is their soul relationships. So to answer your question, wasuke is sukuna's soul twin. Nd if you want to see them as family, then the right term would be gruncle...