(Lk 8:19-21)
Then his mother and his brothers came to him but were unable to join him because of the crowd. He was told, "Your mother and your brothers are standing outside and they wish to see you."

He said to them in reply, "My mother and my brothers are those who hear the word of God and act on it."

· · Gospel Today · 1 · 0 · 2

I have been told that aramaic has one word for both concept of "brother" and "cousin". Is it right?

Yes. A common problem in biblical exegesis is interpretation based on imprecise translation and modern cultural meaning. This is a good example of that.

One (of many) good explanations of this particular issue is here:

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