It's one thing to say "you are part of the family" to the adopted kid and another to effectively state "the blood oath I have sworn to my honored liege lord to serve until my dying breath also includes you".
Opera's formal role is some mix between butler and bodyguard for Sullivan. Technically they have little connection to Iruma aside from him being the guest in the mansion. The oath they have sworn as a "security devil" is a really personal thing and links them first and foremost to their employer. Except Opera does not care about such details.
Sullivan adopted a random (actually not so random, there are theories) human kid and Opera was like "you mean OUR adopted kid?".


















