I’d rather trust the official GOV.UK factsheet than a Guardian article.
Unfortunately, this forum won’t let me post GOV.UK links, but the factsheet says:
"Penetrative sexual activity between people who a reasonable person would think are related, or pretending to be related, in a certain way. This includes depictions of close familial relatives such as father and daughter or brother and sister, and certain step and foster relationships where one person is, or is pretending to be, under 18."
So adult step-family roleplay where everyone is portrayed as 18+ doesn't appear to fall under this specific provision.
Even if only one person is on camera, what matters is how the roleplay is portrayed. If everyone is portrayed as an adult (18+), it doesn't appear to fall under the specific step-family provision described in the factsheet.