YOU SAID:

dad should go to his sister's concert

INTO JAPANESE

お父さんは彼の妹のコンサートに行くべき

BACK INTO ENGLISH

Dad should go to a concert of his sister

INTO JAPANESE

お父さんは彼の妹のコンサートに行く必要があります。

BACK INTO ENGLISH

You must go to his sister's dad.

INTO JAPANESE

彼の妹のお父さんに行く必要があります。

BACK INTO ENGLISH

You must go to the sister of his father.

INTO JAPANESE

彼の父の姉妹に行く必要があります。

BACK INTO ENGLISH

You must go to the sister of his father.