YOU SAID:

My friend is my mum and my mum is my sister and my sister is my cousin

INTO JAPANESE

私の友人は私の母であり、私の母は私の妹であり、私の妹は私のいとこです

BACK INTO ENGLISH

My friend is my mother, my mother is my sister, my sister is my cousin

INTO JAPANESE

私の友人は私の母、私の母は私の妹、私の妹は私のいとこです

BACK INTO ENGLISH

My friend is my mother, my mother is my sister, my sister is my cousin