Why did my sister say that she would marry her brother in Japan?

My sister said that the main reason for marrying my brother is:

Japan can be said to be indifferent to this phenomenon of brother and sister getting married, because many years ago, an emperor of Japan fell in love with his sister. So I told you that brother and sister can get married, so the Japanese have been passing on this marriage custom to the present.

A marriage system of the Japanese emperor's royal family: endogamy. The so-called endogamy, in a word, is to limit the scope of mate selection to a very narrow range.

Compared with free love, the object of marriage can be different across family, private capital, national boundaries and races. Endogamy is a special marriage system, which limits the scope of mate selection.