And step relative sexual activity is prohibited?

This is about forbidding the depiction of such activity, so I don't think logic will help.

There seems to be a nuance (which is missing from the title).

The initial paragraph refers to activities which are prohibited.

    Peers agreed by a majority of one to ban videos and images depicting
    relationships that would not be allowed in real life.

There's perhaps a little more nuance (sexual activity with a step- partner has as an age of consent of 18, rather than the more-typical age of consent of 16)

    They also agreed to bring intimate pictures and videos of adults
    pretending to be children in line with similar images of real children.