I don't know how I came across this, but felt like I needed to respond...
Having myself had a baby that was born with Tetralogy of Fallot (he died when he was 12 days old-born June 2002
)-this was one of the first questions that I asked my OB.
I agree with dawngloves-they do need to talk with their OB. I don't know how your nephew's baby was delivered or what complications his wife had, if any; but that would be the best place to start.
Also, are they in a support group for parents that have lost a baby? I strongly recommend this: there may be other parents that have been or are going through a pregnancy after the loss of a child.