This is a hard one to deal with. I would look at it as tho it is her fiance's child. The only way you will find out otherwise is through a paternity test and considering the hurt that it could cause it's probably not worth it.
Have you talked to your fiance about this yet? Asked him about the condom issue?
I remember a long time ago we had a topic come up on here regarding what folks would do in a similar situation to this...
Here's a couple of topics you might want to refer to:
Pregnancy and Swinging Pregnancy...