Best friend dosnt knw whats right..?

my best frnd is ina relati0nshp since alm0st a year..she n her bf luv eachotha al0adz..they've decided 2 get married in jan 2010..ah was reale happy 4 her..da guy iz reale nice,smrt,caring n evrythn a girl wud want..but i was sh0ckd wen last nyt she disclosd her lyf biggst secret..it waz tht she n her brother had a physical relati0nshp (n0t juz kissin n suckn of eachothaz private partz)..i adviced her 2 tel her bf b0ut this b4 they get married..c0z de r g0in 2 b married n he shud knw evrythn b0ut her lyf..i knw wht she n her bro did waz out of curiosity,lyk she waz 10 n he waz 15..they din knw wht was ryt n wht was wrong..de n0w share a happy n healthy brotha sista relati0n n they dont tlk b0ut it eva..but i dont knw how to convince her..w0t i askd her 2 do waz it rong??hez undrstndn..marriage iza big thing..he has disclosd his entire lyf 2 her..shudnt she do tht aswell? Ply help!waz it right of me 2 ask her 2 do tht..i knw c0nfessin it iza vry tuff thing..tel me sum wayz by which she can tel him..