I thought anti incest laws were to prevent two people who were too close genetically from running the risk of producing defective offspring. In the case of Step relatives this genetic issue does not exist, so the measure for judgement should be age and consent.
I always thought incest was sex between two blood relatives.
Step children are not blood relatives. Although I believe that having sex with step kids is wrong,it is more pedophile rape, than incest, unless those step kids are adults.
If they are adults and consent or even start the ball rolling it is not rape or incest, just bad judgement, and bad morals.