I'm so sorry that you're going through this...
BUT, if your wife had sex 2 weeks before the baby was concieved - then she most likely would not have gotten pregnant at that time. Each woman only has a 24 hour window (every month) to get pregnant. We can't get pregnant every day out of the month. When conditions are right, we have about 1 day.
Most often, a woman ovulates 2 weeks after the 1st day of her period. So, for instance, if your wife started her period on January 1st - she most likely became pregnant between January 12th-15th. She most likely would recieve a positive home pregnany test on February 1st. That's also how the doctor would track conception. If she slept with someone 2 weeks prior to conception, it was most likely right before she got her period on Jan 1st. (which would place her in a very infertile time of the month)
However, because she slept with someone, I would question her honesty of the event happening only once. I would most definitely order a paternity test on the baby. You would hate to devote yourself to this child, only to find out later that you're not the baby's father.
I really hope everything works out for the best! I would suggest family/couples counseling.
they still have dna tests it can prove it one way or the other luck jo