Hi friend! I'm a boy but hope i could at least help u with a few words. Simply if ur bro is older than 13/14 then he definitely knows that it happens to girls. sometimes he may question u "is that going on", but he won't ask more. That's sure. If he is younger ,then he may not notice ur change much but he notices something just simply tell him (i actually dunno how to tell) as @imnayeon said. You know as a brother of 3 sisters I'm totally familiar with this and I know its normal. when my eldest sis had approached it I asked what has happened but next time when it was turn of next sis I simply understood (we were taught in school,isn't it) and did not want to embarrase her. Just take it easy sis. You do not want to hide it from ur bro,one day he will know! Good luck