When Mej was trying to contact the other Commandments, it is revealed that Estarossa survived the battle with Escanor, although having fallen into a coma. Weakened, he manages to wake up, when he overhears Mej ' and Cusack's conversation about the Demon King's decisions about Jem. He asks if Jem is still alive but is quickly interrupted by his brother who advises him to fully recover.
Estarossa used to admire Jem more than anyone, loving his power and ruthlessness. But that love was mixed with burning rage, when Jem’s betrayal lead to the Great War and tilted the balance of power against the Demon Clan, leading to them being sealed. When they finally meet again, he revels in torturing his brother while also crying when delivering the final blow.
Mej is one of Jem two younger siblings. The relationship between the two seems extremely hostile, extremely so on the younger brother's side. While Jem harbors guilt towards Mej, Mej doesn't hesitate to attack Jem and sees him as nothing more than a traitor and an obstacle that must be destroyed in order to conquer Britannia. Not much is known about the history these two share, but it is known that they clashed with one another at some point in the past.
The relationship between the two as it is now is one of hostility. While Jem feels some level of guilt towards Mej when not confronting him, Mej seems to harbor nothing but anger towards his traitorous brother. The exact details of their relationship 3,000 years ago before is unclear but he seems to be unforgiving of Jem’s actions, whatever they may be, in the present day. However, Mej himself apparently views Jem is suffering around Elizabeth and does blame her for his brother's betrayal. Nevertheless, any bond they had has vanished despite Mej apparently having some doubts on his antagonism towards his older brother and finds his actions and betrayal towards the Demon Clan to be unacceptable and unforgivable. Mej also expressed outrage at The Demon King's order to retrieve and crown Jem as the New Demon King. However, due to Jem apparently promising to fulfill Mej's unknown desire that can only be fulfilled by becoming the Demon King, Mej is now working with Jem.
That's all he sent, guess that's the end then byeeee 👋