No, it's not weird to call people "brother" or "sister" even if they're not your literal siblings. The use of such terms is common in many cultures and can signify a strong sense of camaraderie, friendship, or belonging within a group. It is a way to express closeness, respect, and affection for someone you consider to be like family, even if you are not biologically related.
In some communities, people use these terms to address close friends, colleagues, or members of the same social or religious group. For example, you might hear people addressing fellow members of a sports team, a church community, or a fraternity/sorority with these terms.
Using "brother" or "sister" in this context emphasizes a sense of unity and connection, and it can strengthen the bond between individuals who share common interests or experiences. It's a form of endearment and a way to express a feeling of kinship without implying a blood relationship.
However, it's essential to be sensitive to cultural and individual preferences. While many people embrace this form of address as a sign of friendship and solidarity, others may not be comfortable with it or might interpret it differently. If in doubt, pay attention to how others use these terms and adjust your language accordingly to ensure you're respecting their preferences.