I think you ask too much for Yukino,for her she did the right thing and i can't blame her. Yes Nobara seem upset that Yukino didn't try to stop her but it's not like Nobara did drop hints that she wanted Yukino to do it and Yukino didn't want to be selfish about Nobara's future, especially when Nobara said she wanted to study aboard. To be honest, like Yukino said , it's only 1 year. Not like they're gonna be separate forever, I understand what's the point here but it's only 1 year.