User blog comment:Akakitsune98/Does Rin know that Mephisto and Amaimon are his half brothers?/@comment-46.64.148.195-20150924200857

I always wondered that too. I've not seen it mentioned by anyone and I was unsure if Rin knew even though Amaimon said something like 'you could think of me as an older brother"

So if Rin doesn't know I'm going to guess no one else does? seeing as they're not acting upon it?