User blog comment:Akakitsune98/Does Rin know that Mephisto and Amaimon are his half brothers?/@comment-8870791-20120428183810

Well from what we've seen so far in the anime and manga, I think that Rin doesn't know yet.

Im also puzzled as to why Mephisto laughed at Rin for wanting to become an exorcist. Since he is also one.