"Once the law comes into effect, anyone found to posses or publish pornography which shows incest between family members, or sex between step- or foster-relations where one person is pretending to be under-18, will be criminalised...." Wouldn't the step/foster bit already be covered by child pornography laws?
Why would child pornography laws have anything to do with someone pretending to be under-18? That's distasteful, sure, but objectively, people over-18 are not children. Basic recordkeeping laws should make it easy to ensure that everyone involved is of age, even if they're sucking on a pacifier, wearing a diaper, and saying "goo goo ga ga".
CP statutes also deal with "simulations" of underage participants.