YOU SAID:
he was best friend's with his wife's cousin
INTO JAPANESE
彼は妻のいとこと親友だった
BACK INTO ENGLISH
He was best friends with his wife's cousin.
INTO JAPANESE
彼は妻のいとこと親友だった。
BACK INTO ENGLISH
He was my wife's cousin and best friend.
INTO JAPANESE
彼は私の妻のいとこであり親友でした。
BACK INTO ENGLISH
He was my wife's cousin and best friend.
You should move to Japan!