Since the 1960s, Milan once won the championship, and the fans shouted "We are champions". At this moment, international fans shouted at each other, and your champion was cheated. Milan fans are unhappy. At this time, someone shouted, "We are champions, and you are cousins of champions". Soon, Milan fans unified the slogan, "We are champions, and you are cousins of champions". So the name "cousin" was handed down. If you don't believe me, go abroad. This has become a standard name for 40 years.
There is also a saying that Inter Milan was once "bullied" by Milan. When they lost, they complained to the media. The media was nicknamed "cousin" by Italian fans, so it was of course my cousin who cried and complained to "cousin dad".
Why cousins and not cousins? Because the name "INTERNAZIONALE" of Inter Milan is a feminine noun in Italian (Italian is divided into yin and yang).