Brother means relative, fellow Israelite, or fellow believer
Here’s what it means
“Brother” can include a close relative, another member of Israel, or a fellow Christian rather than only a male sibling.
Why the Bible says it this way
An idiom is a familiar phrase whose meaning comes from the whole expression, not from reading each word by itself.
A helpful detail: The wider family or covenant use should not be assumed where a literal sibling fits.