Christians tend to react with equal horror to sex between blood siblings and step siblings. There is good scientific reason for avoiding blood incest — inbreeding leads to deformities and lack of robustness. That does not apply to step incest, between siblings or step-parent and child. There are obviously valid social reasons for avoiding step incest, but it need not be as serious a taboo.
~ Roedy (born: 1948-02-04 age: 64)