Tsuna is the eldest. Chiyo might be his twin, but he's the older brother, and Okaa-san said older siblings must protect their younger ones!
He's the eldest, and he knows that Okaa-san isn't okay. Sometimes he thinks Okaa-san has never been okay, but it's even more obvious after Matsu was born. The only time Okaa-san was okay was when the blond man came, the one Okaa-san said was their Otou-san.
(Tsuna doesn't like him. The warm feeling inside of him, the one that knows the safest way home and the one that warns him away from the weirdos in black, screamed a warning the first time Tsuna saw him. The feeling didn't disappear until Ojii-san poked him in the head, and only cold remained)
Okaa-san won't even hold Matsu! Even when they're crying! It's like she can't even see Matsu. Tsuna and Chiyo have been taking turns taking care of them, but they don't know how to take care of a baby! They can barely care for themselves, and it's unfair to Matsu that Okaa-san can't do it!
Matsu and Okaa-san need help. Everyone in Namimori says if you need help, ask a Hibari, no matter how weird the request is. Namimori belongs to the Hibari, and they take care of their own, and all of Namimori is included in that.
Tsuna is the eldest, and that means he's the one that protects his younger siblings. Even if it means talking to scary Hibari Daichi-san, the head of the Hibari Clan, and asking him to help. He wipes his sweaty hands on his jeans and trades a nervous look with Chiyo. She re-adjusts her hold on a thankfully snoozing Matsu and shoots him an encouraging smile.
Standing on his tiptoes, Tsuna rings the doorbell and stiffens his spine. He might not have his warmth anymore, but he'll take care of his siblings.
No matter what.