Definitely talking (at MOST a kiss). No way those two would do something erotic together... Mikado's way too cold an calculating (except when it comes to Towa)...
Well, two things came up in my mind~
He might meant 'tease' like kiss. (Ooh incest) well there was a chapter, can't remember though, that he kissed Mikado as an example of joke.
Then another one like annoy him again and again about Towa~~ sorry if english is bad orz
hmm probably the normal 'tease' or worse: kissing X3 Okay, Nanao-san might be gay, but I don't think he can do the "thing" with Mikado. Same goes for Mikado~
Its something i wonder about. Plz dont get mad at me, ok. I really LOVE this series<3 But when mikado's brother says he will "tease" mikado, do he mean like talking about towa or tease in bed....??