> But justice minister Baroness Levitt warned that cracking down on pornography depicting sex between step-relatives was complicated, because not all > relationships between step-relatives are illegal.

and then later:

> Lady Bertin said she was "mystified why it does not include step-incest", as she moved her proposal, which peers backed 144 votes to 143.

> She added: "Nearly all step-relations between step-parents and step-siblings is illegal.

> "This is because Parliament recognised the clear power imbalance in step family relationships within households, and also Parliament acted because step-relations are the most likely relationships in which child sexual abuse takes place.

I guess there must be some limited definition of these interactions which are legal.