http://www.thefreedictionary.com/depraved
I don't see that "depraved" requires any element of coercion. However since we don't know the full facts of the case, and what we do know is that the party who ended up on the sex offenders register is the older party, the male, and was suggested to pay for the act then I wouldn't be so quick to rule out any element of coercion.
I'm with you on moral relativism, but I wouldn't say that's in any way specific to this case. Pretty much any time one person judges another we can say that their opinion is subjective. Certainly with regard to the "sex in public" aspect of this case it's easy to cite examples such as the tahitian natives encountered by James Cook's crew who had no such taboo. I struggle to find an objective reason why such a taboo should be in place and yet it is relatively common, the accepted concensus and indeed law in both our cultures even if it is often flouted. I'd happily call sex in public a little bit depreaved though I'm persoanblly happy with a little bit of depravity so perhaps I don't see that as a harsh criticism.
Sex between siblings on the other hand is a far more universal taboo. Again there are pockets of society when the taboo is commonly flouted but here we can see an objective reason why the taboo should persist: inbreeding
WP Here the victim is the potential offspring of the union. Responsible family planning can reduce the risk of such offspring occuring but only drastic measures can eliminate it entirely. Knowing what method(s) of contraception were used (if any) would affect how depraved we'd consider this sort of act to be. However the genetic argument alone is not enough to justify the law prohibiting incest. That would be verging on Eugenics and we know that's a bad thing from argumentum ad Hitler (and pages upon pages of other arguments off topic for this thread) Instead it would seem that the long held justifications for prohibiting cosanguinous relationships exist mainly to place a ring fence around the predetory relationships that characterise many but not all examples.
Another identifiable harm would be the 11 year old under their care being abandoned in a dangerous place as part of this pursuit. Lets not overlook that. Eleven is probaly old enough to be at a train station on their own but forsaking a duty of care with which they'd been entrusted, in order to take part in an illegal activity is still pretty repugnant.