Was Mephisto aware that Yukio had Satan's blood in him after all?

Question

On Episode 6 of Ao no Exorcist, at around 21:30, Principal Mephisto Pheles tells his younger brother, Amaimon, to pass on a message to their other siblings. If the subs were right, it says: "...tell our siblings who are going mad with envy that our youngest brother is being raised under my wing".

On episode 23 however, Yukio offers his own Satanic blood to open the Gehenna gate, and at around 2:40 we can see Mephisto and Amaimon observing the chaos unfolding from above. Being the younger twin, Yukio is technically their youngest brother with their blood - in contrast with their conversation from episode 6. They do not seem shocked by the news that Yukio possesses Satan's blood after all, or they cleverly hid their reaction.

I am asking for clarification, preferably from manga readers: Was Mephisto was really aware of his erroneous statement and was it all really part of his plan or is this merely a (gasp!) narrative plothole of the anime?

Answer 1

I'm not sure if it's a plot hole but it's definitely a bit dubious. Yukio's body wasn't strong enough for Satan's powers, so they rejected him leaving him a pure human.

But Satan is still his dad, so I guess he might still have inherited stuff other than demonic powers. Maybe that includes Satan's blood type, assuming Satan has blood.

There aren't really any answers in the manga, because the anime goes in its own direction after episode 16 or so, but to answer the question, yes Mephistopheles definitely knew he was the son of Satan but that he had no demonic powers, no he almost certainly wasn't talking about him when he said our youngest brother.

Answer 2

Yukio's body was born to be too weak to control Satan's blue flames. But he is still the son of Satan so of course he will always inherit the blood of satan, just not his power.

Answer 3

Mephisto knew Yukio and Rin were twins and Satan was their father, Yukio just couldn’t handle it and proved too weak to show any signs of being a demon. So yes Mephisto knew Satan was Yukio’s kin.