Because it's the right thing to do.

Depending on the disability, yes. If someone has a disability that will basically stop them from living a fulfilling life why would they risk it on passing it on to their children? People with disabilities like this consult doctors in order to see if they are able to have children that won't inherit those genes. It would be morally wrong to put their children through something like that. It's worse to bar someone from truly living their life than barring someone with a disability to have a child that will also have that same disability.

Life created in that way is morally and genetically wrong.

At a moral standpoint, having a child with someone that you are related to will cause the child to get many unfortunate traits that will just cause the child to suffer. Even if life is created, it would not be right as it would live in constant pain as well as the child would not live for very long.

Genetically speaking, having a child through incest is completely against the point of procreating. Sexual reproduction is done so that recessive diseases would be taken out of the lineage and the DNA to become better. It is even makes people want to not have any sort of sexual relationship with family members.

In both cases it would be wrong, but let me know why you think it isn't.