Im Sorry Lisa but you are so wrong in the way you are thinking.
Ultimately, your only responsibility is to your own well being. You are about to enter into a life long commitment with a man who had (possible has) sex with his sister. Instead of worrying about how this will affect them, isnt it more important to think about how you feel first? You obviously are not comfortable with this discovery.
Yes....you need to talk but instead think about the possibility of what effect this will have on your relationship from this point on.
We date to find the person who best serves OUR NEEDS.....not the other way around. Ive had girls break up with me for various reasons and Ive broke up with girl for mine. Ultimately is about YOU and what this person brings to the table.
Now when marriage is involved, it takes all to a new level. You just need to take a breather and think about this.....I suggest seeing a family counselor on your own to discuss the idea of marriage and what this incident will have 20-30-40 years down the road.
I mean, correct me if Im wrong but this is a very serious, serious problem and its just seems from your responses that youre wiling to disregard this incident as a one time thing between to dumb kids.....IT IS NOT. Please get some outside help (other then this thread of course) before, after and during the time youre discussing this with your BF.