Yes! Definitely for AKATSUKI. This ask is interesting to me because as an older sibling, the only guy I can't put myself in the shoes of is Keito. I WILL DO MY BEST, ANON.

For the first part of your question, now that we know that Kanata is (allegedly) an older brother, I think it makes the Kanata/Keito dynamic moving forward funnier. Since this is a recent development, I’m not sure that it can say much about Kanata and Keito’s relationship with each other up to Stella Maris-- as for all intents and purposes, Kanata would've grown up an only child. In their interactions together, Keito tends to be the one trying (and failing) to leash Kanata's lack of inhibitions.

Keito: Shinkai, if you misbehave, you’ll only give trouble to your unit, even your juniors. Get a hold of yourself, you’re incorrigible.
Kanata: Aren’t you bossy~ When my skin gets “dry”, it gets suffocating for me (...) When it was my birthday, everyone “celebrated” with me, yes? I want to "return the favor". And so, I’m “fully motivated” for Scroll of the Elements, as well...～♪
Keito: Hmph. Good to know, then. — Wait, no, it’s not. Get out of the river at once. It may still be early in the morning, but more tourists are arriving…
Scroll of the Elements (TL by citrinesea)

Kanata: Why can't I break the rules sometimes? It's not fun if following the rules is the only way to get "recognized", don't you think? Anyways, I've had enough of this "flower arrangement". I'm gonna "douse" myself in the "pond"...~
Keito: Uninhibited as always. In that case, it's not a flower arrangement but a fish arrangement.
Keito Idol Story, Sole Flower in Various Minds (ENGstars TL)

(This next bit is more of an interpretation/headcanon, based on the content we have of them, so take it with a grain of salt)
●・○・●・○・●
Now that Kanata (allegedly) has a younger brother, him being annoying about it would be funny, especially since Shinkai and Hasumi families have history (Climax, Meteor Impact). If Kanata knows about Keito's older brother, a topic previously irrelevant to Kanata becomes something he can be smug about and pester him with. I think it'd be funny if he brought that energy back with him after Okinawa and seeing Konata. Just smiles, waiting for Keito to ask what he's standing around for.

Keito: 🤨 what’s wrong with you…
Kanata: oh… nothing~ just newfound “maturity”. you wouldn’t get it.
Keito: 😒
●・○・●・○・●

For the second part of your question, we're back to canonical grounding. My immediate thought reading the question was Shinsengumi, where Kuro thinks he can convince Keito to take on a job with MaM.

Kuro: Even if he starts pouting, I’ll see that I make him say yes. He’s a lot smarter than me; he should agree if I explain my reasons (...) Well, sit tight and expect some good news. I’ll go to convince him within today.
(Immediately in the next chapter)
Keito: I refuse. This has nothing to do with you guys. Back off, Kiryuu.
Shinsengumi (TL by kotofucious)

Comparing that to interactions with his sister like in Tough Guy, and it paints a funny picture of two defiant, haughty people doing whatever they want in spite of Kuro's efforts.

Kuro: I don’t see a problem with it… —Yo, what’s wrong? If you’re worried about me, I’m doing grea—
Kuro: Gfuh
Tetora: Ooh!? She did a flying spin kick at you! She managed to hit you right on your neck, despite the size difference! That’s amazing! ☆
Tough Guy (TL by 310mc)

But I think a lot of AKATSUKI's structure as a unit (Keito at the lead, with Kuro and Souma supporting) gets complimented by the fact Kuro and Souma are older brothers. Kuro and Souma are the ones to ground and settle Keito when he gets fussy, stressed or agitated and I feel like they’re good at that especially because they’re older siblings.

Some tangentially related thoughts, in no specific order:
- Souma and Kuro are a few months apart but younger than Keito, yet are older brothers. I just think that's neat.
- Keito says he's relatively independent from his older brother (!! Hinata Idol Story, Practice to be a Little Brother). When Kuro asks him if he likes being doted on as a younger sibling, Keito's like "... what. no." (I can't remember which one this was, I'll update with the source for this later)
- Lots of Souma's big bro-ness shows up a lot with Shinobu.
- Keito has a tendency to look after others, and is usually pretty quick to give in to people when they need or want him to something. I'm sure that ties in somewhere.

Keito: Kiryu, let me support you by the shoulder. Good grief, you’re always so reckless…♪
Kuro: Whenever I’m in bad shape, Hasumi gets unbearably gentle, and it creeps me out.
Chiaki: I think he just likes to take care of a vulnerable kid, or maybe it’s just engraved in his instincts to do so. Hasumi and Tenshouin have known each other since childhood, after all.
Keito: His existence has nothing to do with my development.
Submarine (TL by 310mc)

I would say those are my best thoughts on it, but again... (points at self) older kid. Younger siblings, sound off in the comments below how YOU would pathologize Keito’s “Little Brother Syndrome” /lh. Maybe I'll make a section in the FAQ for stories that characters mention their siblings in. THANK YOU FOR THE ASK ANON!! ❤️☄️