Derrick when i catch you!

You know? They're not blood related, so the fact that Derrick views Penny as a love interest wouldn't bother me if at the very least he was a somehow half-decent person. Sadly, that's not the case -_-.
I find this disgusting not due to the step-siblings issue, but because Derrick is an abuser who never treated her right, and yet he expects to own her just like he owns that beautiful caged bird.

I get what you're saying but they were raised as siblings since they were young. It'd be completely different if she came into the family when they were teens or something because then the raised as siblings aspect is missing. Like if you consider real world adoption from a young age, raised as siblings/your children and then one starts lusting after the other it wouldn't sit right.

Eh? But that's exactly what happened O_O?
The duke didn't adopt Penny as a baby, she was already a grown child of 12yo. It's been six years and just now she's hacing her coming of age. If you consider the age difference with Derrick, he was too in his late teens. That's hardly a child anymore, that's already an age for more mature feelings. And since he never considered Penny his sister, well... I guess that's where his obsession took root from.
I don't remember the age difference between the siblings (was it even stated clearly?), but from their memories, I'd estimate Derrick may be 16yo already when Yvonne disappeared. Reynold perhaps 14yo? So, six years wth sister Penelope for a late teen cannot compare with 16 years (or rather twelve) since childhood with sister Yvonne.
Only my interpretation, though.

Well i get your point but Penelope thought Derek as a brother like her OWN BROTHAER her own sibling which makes it even worse and sad (if my memory is correct though if wrong you can give me more info.)