Uhm so, apparently, Sukuna is Yuuji's uncle.. Yuuji's dad, Jin is actually reincarnated version of Sukuna's TWIN brother. Which, Sukuna ate him when, both were still in the womb to survive from starvation. Which later, made Sukuna an unwanted child later on.
Kenjaku got a hold of Jin's soul. Reincarnating him and as you know, having Yuuji later on with him under Kaori's body.
MEANING, Sukuna and Yuuji are somewhat if fully, RELATED. Kenjaku being Sukuna's Sister/Brother in Law.
Jin is Yuuji's father (Bio). He dies when Yuuji was just a baby so, we didn't really know much about him until very recently.
He is reincarnated version of Sukuna's TWIN brother. After Jin died, instead of you know, going to the afterlife and all, Kenjaku got a hold of his soul and reincarnated him.
As you know, Choso and Yuuji are brothers. But, they don't share the same father so, Sukuna is Choso's Step Uncle? I can't fully explain. It is complicated asf and, took me a good while, to fully grasp all the information because you know, JJK is complicated and especially MESSY.
It is messy and complicated but that just JJK, haha. Yuuji was never a really human at start. I mean, his strength is way above an average human and of course, his speed. His origins were never really fully explained explictly. Until, really now.
Yuuji still doesn't know about his own origins
Which, is lowkey crazy.
Bro, bring back season 1, I don't want a fucking love story. ☹