@Anonymous

In Japan, when you're familiar/friends with someone, like in this case a younger sister and are made to interact on less-than-friends relations with someone else, it's common to refer to that person by stating his/her relationship to your friend – it's like in anime you find someone saying per "[friend's name]'s mother" when visiting home or meeting on the streets. In this case it's like an abbreviation of "Sakurako's elder brother". It's similar to calling anyone you're not in defined close-friends relationship, by his/her surname, not given name – you find that they always use surnames while speaking (at least this is how it always was – can't say anything about kids nowadays). Of course this applies to well-behaved speakers.

Etiquette in Japan is pretty important, when I had a Japanese visiting me here on summer, he actually bowed on the city market to my Japanese teacher apologizing for being late even though they met for the first time. And I mean the bow you can see in anime.

PS: I'll try getting busy 😉 I wanted to make few chapters this week, but I'm not sure now – my driving licence classes are distracting me a little.