In the world of Demon Slayer, the Ubuyashiki family has long been plagued by misfortune and tragedy. As the head of the Demon Slayer Corps, they carry the weight of the responsibility to eradicate the powerful demons, especially their progenitor, Muzan Kibutsuji. However, their family history is marred by a supposed curse that prevents any Ubuyashiki from reaching adulthood. Many have attributed this curse to their blood relation to Muzan, but there may be a more logical explanation for it. Perhaps the Ubuyashiki family isn't necessarily cursed for giving life to the progenitor of demons; they are simply sick.
Source：https://www.cbr.com/demon-slayer-theory-ubuyashiki-not-cursed-just-sick/
