I'm interested about the fact that the sister said they agreed to dress "appropriately", and the fact that he seems rather reluctant to agree.

I'd suggest in order to agree to it, he may demand them wearing and doing stuff that make it worth the bother for him.

We've seen he's interested in sex, but unwilling to do it to his little sister.

He may demand they service him sexually before they get to enjoy his cock. And probably he makes them wear something a bit humiliating.
Another idea could be to make them wear something they cannot take off without causing damage (butt plug with hooks, or something like that ?). This could be a setup for a followup story where he gets to complete their outfit in a similar spirit as the plow story, transforming them into an animal or an object.

Actually that made me think, I'd love to see a forniphilia arc at some point, especially now that we've introduced more characters it makes it easier to have fun with.