YOU SAID:
Will you marry my brothers cousin?
INTO JAPANESE
私の兄弟のいとこと結婚してくれませんか?
BACK INTO ENGLISH
Will you marry my brother's cousin?
INTO JAPANESE
兄のいとこと結婚してくれませんか。
BACK INTO ENGLISH
Will you marry my brother's cousin?
That didn't even make that much sense in English.