User blog comment:Akakitsune98/Does Rin know that Mephisto and Amaimon are his half brothers?/@comment-8870791-20120428183810/@comment-1948423-20120917225421

Well it ccould be because Mephisto is a full demon, has control of his powers and has some from the Order. While Rin still doesn't have control of his demon powers and is not as experienced as Mephisto and the Blue flames show his relation to Satan.