YOU SAID:
brother found out his sister liked girls
INTO JAPANESE
兄は妹が女の子が好きだと知った
BACK INTO ENGLISH
My brother found out that my sister likes girls.
INTO JAPANESE
妹が女の子が好きだということを兄が知りました。
BACK INTO ENGLISH
My brother found out that my sister likes girls.
That didn't even make that much sense in English.