Well, I am a native Japanese speaker, so you don't need to tell me how the term '兄弟(brothers)' is used in Japanese

. Dictionaries don't tell you everything about how a language is actually used.
It is true that '兄弟(brothers)' is used to refer to siblings in general. For example, if someone asks you, 'ご兄弟はいますか?(Do you have any brothers?)', you might answer, '弟が一人と姉が一人います(I have one brother and one sister).'
However, using the term '兄弟(brothers)' to describe yourself, as you would with a T-shirt, is a different matter.
It's difficult to explain why this feels strange unless you're a native speaker, but labelling (implying) Maia as a '弟 (youger brother)' just doesn't feel right to me. If they wanted to demonstrate unity by using their family name and relationship, they could have used 'チーム渋谷(Team Shibutani)' or even '渋谷家(Shibutani Family)'.
The term '兄弟(brothers)' also carries connotations of masculinity and is used in the Yakuza world to show the hierarchical relationship between senior and junior members. The younger brother must obey and listen to the older brother. Although I am very certain this is not the intention behind using this term on their T-shirt, in light of what I have seen lately, that is another reason why I found it uncomfortable.
While this is really not the same situation, when I came to the US many years ago, I sometimes got letters addressed to me using my husband's full name, 'Mrs George Washington', rather than 'Mrs/Ms Yuki Nieve' (by the way, this is not my husband's real name). Do any of you remember those letters? It's not conceptually wrong, but I found it very strange. Seeing '兄弟(brothers)' on a The T-shirt reminds me somewhat of a similar sense of incongruity — it's not really 'me'.(and fortunately I no longer receive such letters).
Come to think of it, many non-native Japanese people have been putting kanji (Chinese characters) on tattoos or shirts lately, thinking it's cool. I think the Shibtanis may just be using them for the same reason.