Many birth defects are rare, and require 2 copies of a defective gene to show up. Most “normal” people will be carrying a few defective genes (out of thousands of pairs), but are fine because they have a good copy still working.

Family members tend to have similar genes.

The chances of you and a family member having the SAME defective gene are massively greater than you and some random stranger.

Thus any child would also have a massively greater chance of inheriting 2 identical copies of the defect.