@givemersspls "The reason that the arc even stands out is not because of brother and separated sister but instead because of Toru and his sister, the interactions between two established characters barely at all related to the "main events" of the arc. "
I think you've hit upon the real thrust here. The point of this mini-arc was to turn the spotlight on to Toru and his sister. The "main events" feel shallow because they aren't really the main events.
The "friend list" thing is actually really interesting. To me, it hints at Toru's mindset, and the psychological baggage he carries that we don't see from the outside. We know he's lost all his friends before when he was outed as an Iris Zero, and suffered at the hands of people he once considered friends. We know he intentionally tries to not stand out to avoid a bullying problem that's abated over the last decade, but never really STOPPED.
I suspect he's intentionally keeping his not-friends at arm's length as a defense mechanism.
If he doesn't have any friends, he can't lose his friends again. He can't be betrayed by his friends again. He can't be hurt by his friends again. It won't matter when (not if) they turn on him.
From Toru's perspective, he can't afford to consider them as anything more than some people he hangs out with sometimes.
From another perspective, he simply isn't capable of trusting another person enough to call them a friend.
Basically, Toru's sister knows he doesn't have friends because she's seen his friend list and she knows it is empty But what she doesn't see is that he has a bunch of friend requests he hasn't accepted yet.
If you ask me, this is setting up for a series finale. Toru's helped everyone else, now it is time for everyone else to help Toru.