MC confesses to his childhood friend and they become a couple.
His yandere imouto proceeds to murder them both and then off herself.
MC is reincarnated as the next demon king in a world where humans and demons are on the cusp of brokering a lasting peace treaty.
His childhood friend and sister turn out to have been reincarnated as princesses of the largest human kingdom, as half-sisters too ironically enough.
Childhood friend knows who the sister really is but has kept mum about her own identity because she's terrified of being murdered again by her. This situation is compounded by the fact that the sister was born with an absurd amount of cheat powers signifying the hero's bloodline in her. The two of them are sent by the king as an envoy to the demons when they come of age.
In the land of the demons, MC and his childhood friend reunite and discover who each other are. The childhood friend begs him to protect her from his crazy sister while the sister independently discovers the MC's identity while sneaking around the castle as a spy and rooting through his personal belongings.
And thus begins the war between humans and demons once more when the Hero bursts into the throne room screaming "AT LONG LAST WE'VE REUNITED ONII-SAMA! YOU'LL NEVER LEAVE ME AGAIN!" while waving around her holy sword (named after her beloved Onii-sama of course).
She slays the current demon king, MC's father for "getting in the way between me and onii-sama", but at the last moment, he manages to suicide bomb her with magic that sends her flying into the stratosphere and onto another continent (she survives of course).
Humanity then declares war on the demons once more for attacking the Hero and kidnapping her sister. The MC and his childhood friend are faced with the impossible task of returning peace to the world, all while trying to stop the absurdly overpowered, batshit insane Hero.