YOU SAID:

I am going to be a brother

INTO JAPANESE

私は兄であるつもり

BACK INTO ENGLISH

I'm going to brother

INTO JAPANESE

弟をつもり

BACK INTO ENGLISH

My brother is going to

INTO JAPANESE

私の兄のつもりです。

BACK INTO ENGLISH

It is going to my brother's.

INTO JAPANESE

それは私の兄のつもりです。

BACK INTO ENGLISH

It is going to my brother's.