Forcibly take the child into care once it is born - deny all contact with the parents.
From the child's point of view this is horrific; not only will it's fucked up gene-pool possibly result in God-alone-knows what deformities, if ANYONE finds out about this (as they already have) he/she's liable to be bullied to the point of suicide.
If these incestuous freaks wanna bump-uglies, fine, whatever - but don't bring a fucking child into it; inconsiderate bastards.
EDIT - I failed to take in the part about the donor egg etc. etc. so I don't have quite so much rage to vent, but still, if anybody finds out about that child's "parents" when he/she's at school or college, my point about bullying and a life-time of abuse still stands - children won't make the distinction about donor eggs, all they will see is granny-banging.