It would take a long time, and it's possible that the children and grandchildren could get some pretty bad genetic disorders, Dominant and Recessive, since they're marrying closely related family members, most likely. I mean, look at the Royal Families in Europe and Russia. Many of them had Hemophilia, and passed it on to their children, since they married carries or someone who had it.
Plus, how would they know if they were a carrier? There's no geneticists left to tell them. :/ Lucky guesses?
Plus, there's a chance that the first mother could die in the labor process, or have a miscarriage.
It's possible, but it would there's loads of risks. :/
Too bad there isn't 2 girls. Make things a little better.