YOU SAID:

My brother is the brother of my cousin but only in parties where none of them are present

INTO JAPANESE

私の兄弟は私のいとこの兄弟であるが、それらのどれもが存在しない党でだけ

BACK INTO ENGLISH

My brother is my cousin brother, but only at a party where none of them exist

INTO JAPANESE

私の兄弟は私のいとこ兄弟です、しかし、それらのどれも存在しないパーティーでだけ

BACK INTO ENGLISH

My brother is my cousin brother, but only at a party where none of them exist