....didnt you know? Its Yaoi Logic duh....
I think they mean they're in the same year in univ, not the year they born.
I think it's lost in translation ... most likely both siblings were born within the span of a year but they have been treated as having the same age...
As I understand it, Hinata(?) is born in april and his sister in march the year after. But because of how the school system is in Japan they ended up in the same class(year of school). It's really confusing but in Japan the ones born early in the year are put in the same class as kids born during the middle/last part of the year before.
Oooooh that makes so much sense now :O thank you op!
Glad if I could help. ^^
Sorry if I got something wrong though, I don't fully understand it myself.
Wait... did they just say the siblings were born in the same year? It would make sense if they were at least 10 months apart... but one was born in April and one in March? How does that even make sense lmao