In Japanese, 'a family' means family members who are in the same household, and "cousins" are not 'brothers' or 'sisters.' But in some other countries, they call cousins brothers or sisters. It is sometimes difficult to know which meaning they are using when talking about their siblings.
One day, a new student brought a man and introduced him to me, saying, 'This is my brother.' I had no doubt that he was the student's real 'brother,' since the student said that he had studied Japanese more than two years. But as we went on talking, he said the man with him was the same age. Then my brain began to work busily. ---The same age!? But they don't look alike at all. Do they have different mothers? How do I continue this conversation?--- I was very upset, but afterward, it turned out that they were cousins.