Why does in-breeding cause birth-defects?

Answer 1

as hot as you think your sister might be, consider the following first

if your family has a recessive gene that causes an abnormality, that recessive gene would possibly become both the recessive and the dominant gene in a child

because such a gene is recessive in your family, it is not seen, and usually a healthier, more dominant gene took charge when you were created

Answer 2

When you make a baby it chooses between yours and hubbys genes. If you and hubby have the same faulty gene then the baby will have no choice but to have a faulty one. If you are related then you more than likely will have the same genes and therefore not give your child any good genes to choose. I hope I answered your question.