I think so too, the Older Brother can clearly deal with Shia, but seems she overall has issues with People who are not her Friends, or Takusu/Tsugumu family(or so does seem to me) so now the the Older Brother sees who easy He can be around her and deal with her, he is amazed and ponders if maybe they are already going out. Given how Takusu has gotten so close to her it must be very impressive to the outsider, how Quickly Takusu went from 0 to a Shia Connosieur. (besides he would be amazed if they indeed were going out, he is 13 and she is 16, if my younger brother in middle school got himself a Girlfriend who is my classmate in highschool i would also be amazed)
Their names are spelled Tasuku and Tsumugu, by the way.
Also, Tsumugu isn't in the same class as Shia. In fact, I don't recall it ever being made clear how old he actually is, but I feel like he's at least around university age, if not beyond. That is, I'm pretty sure he's at least as much older than Shia as Tasuku is younger than her, if not more. The only one that's maybe around Shia's age is Shizuku, their sister, who we know for a fact goes to the same school as Shia.
But you do have good points about how well Tasuku is handling being associated with Shia, and that Tsumugu seems to be taking note of that. The fact he even thinks they're dating is quite telling. Like, seriously, I think just about everyone thinks they're an item already, even Tasuku's classmates, with how that last chapter went down.