If you are engaged, would you and/or your family attend your fiance’s brother’s wedding because you aren’t officially married yet? – Gurvinder
If you have been invited, you and your immediate family should attend you fiance’s brother’s wedding. He is soon to become your brother-in-law and his wife, your sister-in-law, so it is only appropriate that you be there at their wedding. After all, they are part of your future family.