Yuji is Sukuna's son, not nephew.

Okay, this idea isnt as controversial as it would have been a month ago but we have more evidence now. Yuji is the son of Sukuna, physically, and spiritually.

First and foremost, we have confirmation by Sukuna that Jin Itadori was the reincarnation of his would-be twin. Sometime during the pregnancy, Sukuna ate his womb-mate, thus denying the fate of a twin from ever happening. We have seen how the Jujutsu system treats twins, Mai and Maki. Sukuna would have been born weak, and likely never reach the peak he now enjoys. By cannibalising his twin, Sukuna wrestled full control of their soul to himself.

Within the Jujutsu World, twins share the same soul. This we all know, based on Maki attaining her awakening post Mai's death. As such we can assume the relationship between Jin and Sukuna like this: Jin Itadori's soul = fragment of Sukuna's soul. Thus we can connect Yuji's spiritual lineage to Sukuna through Jin.

On the other hand, Kenjaku ensured that Yuji would also inherit a portion of Sukuna's physical power, to better serve as a vessel, and prison, and we should have seen this coming. We know now that Kenjaku had "sealed" a Sukuna Finger within Yuji from birth. In fact, we've seen Megumi call it out.

Chapter 1: Megumi called out that Yuji felt like the Curse Finger, but he didnt have any with him, right? At that point, Yuji did have one inside of him already, this explains exactly why he was so physically powerful even without CE manipulation. Sukuna's CE was woven into his very dna.

This conveniently explains why Yuji could survive the possession and the inherent poison of Sukuna, Yuji had resistance to Sukuna because he shares similar CE.

It was there since the very beginning.