Plz advise me what to do?

i am the youngest of four sisters.My second eldest sister is getting married tomm,we are so happy for her bec she says she,s never been so happy in her whole life and that she feel like a queen with this guy.Last nite i saw my third sister ******* my future brother-inlaw.They don,t know i saw them.Ilove all my sisters,and i know definitly this will brake our bond if i must say.What do i do,it,s tearing me apart.they act like nothing happen.How long has this been going on.