persian-slipper answered your question: Omegaverse Headcanon Question Time!
What does Mycroft think of John’s status as an omega, especially since he lives withe alpha Sherlock?
Honestly, Mycroft had pretty much written off the idea that Sherlock might ever end up settling down with anybody. He was never terribly invested, either way, but he is/will be ASTOUNDED to see Sherlock and John together as, like, an actual serious couple.
Of course he had John thoroughly checked out when he first moved in with Sherlock, just like he did in the show. He was somewhat concerned by the potential for mess that their arrangement created, but seeing how seriously John takes the whole thing, his concerns for the consequences of stupidity were largely allayed.
Which is not to say he didn’t stalk the hell out of them, of course, just to be sure.
He did start to get significantly more worried when Sherlock began to take what seemed to be a sexual interest in John. I mean…well, he’s just got no damn idea what he’s doing when it comes to sex, does he? And John…Mycroft’s not sure whether Sherlock’s noticed, because he ignores that sort of thing, but it’s pretty clear to Mycroft that John’s in love with Sherlock, so. Yeah. That makes him nervous.
It’s really the love that has Mycroft disturbed. Long-term consequences can ensue from bad decisions re: sex, but love can destroy people.
And he isn’t really clear on how Sherlock feels, or if he’s thought about any of this, or if he’s working on any kind of rational process at all (and worse, he knows that the reason he doesn’t know is because SHERLOCK doesn’t know).
So yeah. At first, he was mildly concerned but then not really, the better he got to know John. And then he got more concerned again, and now he’s actually kind of quietly terrified for his brother, because with these emotional entanglements, this is playing with fire.
And since you asked, now you’ve got me pondering whether he might decide to intervene somehow. That sort of thing is always risky on matters of the heart, but…this IS family.
Which didn’t exactly answer the question, did it? ^_^ Regarding John’s status as an omega, he’s very impressed that John’s come as far in life as he has. Mycroft 1: understands that omegas with careers face a lot of difficulty and 2: he does somewhat fall prey to ideas about what omegas are and aren’t cut out for. But ever since John stared him down in the warehouse, he’s known better than to make assumptions about John.
(That, incidentally, pisses John off. He shouldn’t be a fucking exception; it’s a basic human courtesy every omega deserves as much as anybody else does.)