You are correct in that both “brother” and “sister” have a multitude of meanings. Strongs concordance (79&80) lists siblings first, the other definitions follow so context in reading is very important. See Matthew 13:55-56.King James Version (KJV)
55 Is not this the carpenter’s son? is not his mother called Mary? and his brethren, James, and Joses, and Simon, and Judas?
56 And his sisters, are they not all with us? Whence then hath this man all these things?
So it begins with talking about Jesus’ earthly father, then about His mother, Mary, then about close friends or cousins? Nope , the reference was to siblings, upheld by all except for the RCC which does this to justify their Deity status of Mary. Sorry, your argument about brethern (same Strongs # as “brother) does not stand the test of scriptural study.
Sorry, your argument about brethern (same Strongs # as “brother) does not stand the test of scriptural study.
If Mary had other children why would Jesus appoint John to be her son?
It is just an assumption that Mary would be alone other wise, but it is a pretty good assumption.