"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?

I'm pretty leery of criminalizing possession here, especially if it's material produced before the law was passed, but even in general. It's not like the material is depicting actual abuse, where you can argue the participants would be harmed by people watching and sharing the material.

Sounds like going forward the way around this would be to emphasize in the script that all the characters aren't related by blood and that everyone is of age.

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".