Fami will offer to transfer some of Denji's lifeforce into AsaYoru to heal them.
Denji is immortal so it won't kill him.
But Denji will have to kiss Fami for the transfer to work.
Asa and Yoru will have to watch Denji make out with their sister just like Nayuta watched Denji make out with them.
It's like poetry it rhymes.