Responses
It runs in the family, don't worry about it.
Their Grandfather and Father are also in a sexual relationship, Sekai and Mio feels mild in comparison. From what I can tell, Grandpa is an unaging 28 year old in his 70's. I don't know if they needed to consummate, or if there's some supernatural conception of holding hands to birth a child.
wait wtf the author cant just drop a "the older bro leads a satanic cult and that's why he's in an incestuous relationship with his younger brother" bomb and not elaborate