YOU SAID:

My brother's mother is the daughter of my uncle's father. My sister's daughter is the cousin of her uncle's son. My son is the grandchild of my cousin's uncle's father.

INTO JAPANESE

私の兄の母は私の叔父の父の娘です。私の妹の娘は、叔父の息子のいとこです。私の息子は私のいとこの叔父の父の孫です。

BACK INTO ENGLISH

My brother's mother is the daughter of my uncle's father. My sister's daughter is my uncle's son's cousin. My son is the grandson of the father of my cousin's uncle.

INTO JAPANESE

私の兄の母は私の叔父の父の娘です。私の妹の娘は私の叔父の息子のいとこです。私の息子は私のいとこの叔父の父の孫です。

BACK INTO ENGLISH

My brother's mother is the daughter of my uncle's father. My sister's daughter is my uncle's son's cousin. My son is the grandson of the father of my cousin's uncle.