Uhh.. That's a complicated way of seeing things. Let me think about it for a bit
Hmm... Let me use point base form to specify the relation between those character.
Kiel - Rood = Brothers by blood.
Demon Rood - Demon Bro = Brothers, but i think they're not brothers by blood.
Rood - Demon Rood = Coexist together interchangeably in one body
Demon Bro - Meredith = I assume they too coexist together interchangeably in one body. One of the reason i point out is, normal human can't simply able to live for a very long time.
Meredith - Roods = No relation
And to extend on the matter.
(@theory)
Meredith - He is a human in the past, but might've coexist with demon bro. Allowing him to be able to live for a very long period of time
Lispen - Splinter group of Demonic Realm. Wonder is he part of the demon responsible for the invasion of human realm in the beginning of dawn of magic for human in BH.
Demon Rood - Almighty king of demon realm. Something must've happen in the past that results in him being what he is now, and demon bro being what he is now. That thing might be the upcoming super villain in the future