With the big reveal in Jujutsu Kaisen chapter 257, the fandom has been sent into a frenzy. Many previously held theories about Yuji Itadori and Ryomen Sukuna being brothers have been shattered, as almost no one expected Jin Itadori to be involved in the grand scheme of things.
However, the Demon King himself explains everything at the beginning of the chapter. With seemingly all the information now on the table, there is an inkling that Sukuna had this coming for a while. It is possible that Jin Itadori had planned his brother's downfall long ago, possibly even from the time Yuji was born.
Jujutsu Kaisen: Jin Itadori could have masterminded Sukuna's downfall
At the beginning of Jujutsu Kaisen chapter 257, Sukuna reveals to Uraume the truth about Yuji Itadori's real identity. As he explains, the King of Curses was meant to be born a twin. However, given twins being considered a bad omen in the series, Sukuna consumed his brother in the womb.
Consequently, his twin's soul roamed aimlessly until it fell into Kenjaku's hands. Through some method, the soul was then reincarnated as Jin Itadori. From here, it is understood that Kenjaku took over Kaori Itadori's body, procreated with Jin, and thus birthed Yuji. So, as Sukuna explains, the boy possesses a part of his soul and a similar latent potential for Jujutsu.
This revelation leads to speculation about whether Jin could have planned this all along. Referring back to Jujutsu Kaisen chapter 143, Yuji has a flashback of his father, mother, and grandfather. In the scene, Wasuke (grandfather) warns Jin to leave that woman (Kaori/Kenjaku), otherwise, he would die.
Jin responds by asking his father not to speak that way since "babies remember more than we think." This line is particularly intriguing, as Sukuna recalls being in his mother's womb and actually consuming his twin, it is very likely the twin was aware too.
This would explain his soul bouncing about, most possibly searching for a suitable host. Kenjaku intervening may have been a godsend for it, since it reincarnated as Jin. The former must have realized the implications of all this and systematically took over Kaori's body.
Knowing all too well that Jin and Sukuna's souls possessed similar properties, Kenjaku could have proposed a deal - reincarnation in exchange for birthing Yuji. It wouldn't be wrong to say Jin may have agreed, possibly to exact revenge on his brother for what he had done to him.
Both parties (Jin and Kenjaku) were likely aware of what the offspring of their union would be - a child possessing the aptitudes of both Kenjaku and Sukuna. This is probably why he uttered that sentence, "babies remember more than we think," feasibly referring to his recollection of being consumed.
Moving forward in time, standing true to the line, Yuji does have a flashback. As seen, this flashback is exactly that moment, when Jin converses with Wasuke and they are interrupted by Kaori/Kenjaku. In all likelihood, Yuji is unaware of his relations to Sukuna and Kenjaku.
However, due to the amalgamation of their influences within him, Yuji's affinity for Jujutsu, evolution rate, and potential have skyrocketed. He has shown massive growth, awakened two Cursed Techniques (Blood Manipulation and Shrine), and shattered the record for most consecutive Black Flashes (8).
Final Thoughts
Truly, creator Gege Akutami has hit fans with a curveball that has left everyone reeling from the impact. With almost everyone certain that Yuji was Sukuna's twin, Gege threw a spanner in the works and revealed it to be his father, Jin Itadori. This was a twist no one even dreamed of.
Despite this revelation, there are some things yet unclear that may be resolved as the story further progresses. For instance, how did Wasuke know Kaori/Kenjaku was trouble? Was it these facts that he wanted to reveal to Yuji at the very beginning? These are still some questions that need answering.
Also, there is another bit that cannot be ruled out - did Kenjaku fill Jin in on everything and then plan out having Yuji? or deeply saddened and in denial of his real wife's passing, he agreed to Kaori/Kenjaku's request for fear of not losing her again?
Related links:
Jujutsu Kaisen's confirmed Twin theory in chapter 257 drags Jin Itadori into the spotlight
Yuji Itadori's family tree following Jujutsu Kaisen's twin reveal, explained