A.C.D.C.2 GTS manga

[j2k]

Originally Posted by chachazero90

So they`re siblings, the last page makes me all fuzzy inside.

I don't know a single bit of Da Capo, let alone the relationships among its characters, but I'm sure you know that even if "onee-san" and "onii-san" mean "big sister" and "big brother", they're commonly used to refer to people who you share a strong bound with even if they're not your actual brothers or sisters.
Actually, I don't think it's so common in spoken Japanese. But it is in manga.