Wow. This is very nice.

Sorry dude. I'm sure dealing with this must be very difficult. But you shouldn't have to deal with it alone. You claim to love her and you say she completes you, yet you couldn't tell her something as serious as this.
My advise though, don't go through with the marriage. Find a way to end it quietly. If the treatment does work and you have children of your own, you'll never love that child she's pregnant with. You'll always hate and detest the child and maybe one day you'll even end up telling it to her face and then the marriage will be over.
What if the child doesn't look like either of you, then one day, someone just comes into the house to see your wife and you look and just know that he's the father.
Don't do something you'll regret. You'll never be happy. Confront her about it now.