The marriage between a mother-in-law and son-in-law has become increasingly complicated, and I have no doubt that things will be even more complicated in the second half of the relationship. My son-in-law is a blood relative, but we are not blood relatives. While we do share some DNA, we are not related. That's why we can't have sex without being related. This is what's happening between our mother-in-law and daughter-in-law..