Yeah I know that it's not Yuji but Wasuke who was the reincarnation of Sukuna's twin, but I don't think that rules out my hypothesis.

Let me first state the reasons why I think Yuji and Sukuna have the same soul.

|-->Uraume and Sukuna's conversation in ch257.

Uraume feels there is a similarity between Yuji and Sukuna, that cannot be explained by just the leftover traces of Sukuna's CE in Yuji, and Yuji is like an empty husk which can actively retain elements from Sukuna.

Sukuna answers by first guessing that Kenjaku's mate was his reincarnated twin, and then says "Our Souls are the same".

If you only look at this one speech bubble, I can see why it would seem as if Sukuna is talking about his reincarnated twin when he says "Our souls are the same", but if you look at the entire conversation, I think it's more clear that when Sukuna says "Our Souls are the same", he is talking about Yuji.

Sukuna saying "Our Souls are the same" is the answer to Uraume's query about Yuji's resemblance to Sukuna. Yuji's and Sukuna's souls being two fragments of the same soul is what produced this resemblance.

It would be redundant for Sukuna to first mention his other half and then again say 'Our souls are the same'.

Moreover, I think in the original japanese, Sukuna's statement is closer to--

'I judged/deemed/saw the soul to be the same'

So In my opinion what's being conveyed here is that Sukuna judged Yuji's soul to be the same as his reincarnated twins and by extension the same as his, which is what leads to the resemblance.

||-->Resistance to Sukuna

I think it's fair to conclude that the resistance of an individual towards Sukuna depends on their soul.

This is why Sukuna waits until Yorozu's reveal (which 'shatters' Megumi's soul) for switching to Megumi's body.

This is why he has to do the whole Bath ritual for weakening Megumi's soul.

So Yuji's ability to supress Sukuna must also be tied to his soul.

This fits perfectly with the idea that Yuji and Sukuna share the same soul. Yuji having the same soul as Sukuna makes it essentially impossible for Sukuna's soul to 'be stronger' than Yuji's in any way or for Yuji's soul to be vulnerable to Sukuna's, which is what makes Yuji a 'Cage' instead of a 'Vessel'.

|||--->It's consistent with a lot of things.

--->It fits with the idea that Kenjaku engineered Yuji for the purpose of Sukuna's incarnation, as Yuji having the same soul would-

1) Guarantee that Sukuna's incarnation, bypassing the 1 in a          million chance.

2) Make Yuji immune to the poison that is Sukuna's finger. Yuji having the same soul as Sukuna, is why he is able to live despite having Sukuna's finger sealed inside of him.

--->It fits with Yuji having the same latent potential as Sukuna. While familial bonds do hold some  in JJK, with the entire concept of Clans and hereditary techniques, I don't think 'Latent Potential' being linked to ancestry has ever been present in the series. So this statement makes much more sense, and feels more justified with Yuji having the same soul as Sukuna.

--->It fits with Mahito aggravating Sukuna, when trying to touch Yuji's soul. Ofcourse the simple explanation is that Mahito cannot touch Yuji's soul without touching Sukuna's soul, as they reside within the same body, But if you think about it even Yuji was able to perceive the boundary between Sukuna and Megumi's soul, and strike at it, so it seems a little far fetched that Mahito who 's entire CT revolves around souls wouldn't be able to differentiate between Yuji and Sukuna's souls.

The truth is that, you can't touch Yuji's soul without also touching Sukuna's, because both of them are fragments of the same one soul.

--->This idea also fits better with Sukuna saying that Yuji was from 'back then', otherwise the statement doesn't make much sense.

•

•

•

So now the obvious question is-

"How can Yuji have the same soul as Sukuna if Wasuke is the one who is Sukuna's twin's reincarnation?"

I can think of two possible explanations--->

|•••>
I think the answer is Kenjaku and soul fragmentation.

Souls can be divided into fragments. It's a part of Sukuna 's origin--him consuming his twin splits his soul into two, and then even after his death, his own part of the soul is further split into twenty different parts.

This might be a special phenomenon only possible with Sukuna's soul, but nevertheless we know that it's possible.

So perhaps, when Kenjaku stumbled on Wasuke, he realised that this was the reincarnation of Sukuna's twin, and from then on he worked to ensure that he wouldn't lose access to Sukuna's soul.

Using his soul-breaking skills, perhaps he was able to fragment Wasuke's soul, and pass it onto Jin, and then subsequently fragment Jin's soul to create Yuji.

So yeah what I'm trying to say is that Jin was also just  'a Yuji'  created by Kenjaku by procreating with Wasuke. It's just that time wasn't ripe enough, and he didn't need Sukuna to be incarnated during Jin's entire life.

I say this because, from what Sukuna says in ch 257, it seems to me as though 'mating with reincarnation of Sukuna's twin' was a required part of this whole 'Create a Yuji' endeavour. 

I believe this is also supported by Wasuke's whole demeanor about Kaori. The whole 'Yuji as a baby' flashback scene, to me seems to imply that Wasuke knows more about 'individuals with forehead scars' then he was letting on.

The thing is, that he very ominously says "You'll die".

I understand, that a woman coming back after her death with scars on her forehead is indeed very dark and ominous.

But, instead of saying something like 'She's cursed' or "She's strange" or "That's not your wife", he straight up says "You'll die".

Maybe, I am overthinking this, but this whole scene just doesn't sit right with me, if I assume that Wasuke knew nothing about Kenjaku, and was just warning Jin because he found not-Kaori's situation paranormal.

The existence of the disparity between Sukuna's picture of Jin being the reincarnation of his twin's soul, and Wasuke being the actual reincarnation of his twin's soul, imo also motivates this.

So in conclusion Wasuke, Jin, Yuji and Sukuna are fragments of the same one soul.

||•••>

The special 'messing around' Kenjaku did with Yuji when he was a foetus, was actually engineering him such that he is able absorb and assimilate cursed objects.

Along with this he sealed Sukuna's finger inside of Yuji before he was even born, so Yuji has been assimilating Sukuna's finger (which houses a piece of Sukuna's soul) which is why he has gained a soul which is practically the same as Sukuna.

His ancestry and his grandfather being Sukuna's reincarnated twin was probably necessary to ensure Yuji's compatibility to actually be able to assimilate Sukuna, and similarly Yuji was able to absorb/assimilate the death Painting Wombs because of Kenjaku being a common parent between them.

I personally like the first explanation more.

So What do you guys think?

Yuji and Sukuna are fragments of the same soul.