it's incest independent of blood
by law its still incest. arguably, theyve always been raised as siblings, though derrick obv doesnt see her as one lol.
ah eclipse, as always every time i remember he betrays Penelope my heart aches.
That's why he despise (Derrick) himself for feeling that way towards her thus he distances himself. Although in the long run, he wouldn't help Yvonne once he wake up from her manipulations.
True. Only if his mindset was different, I would have been rooting for him. Unfortunately, that's not the case.
When Yvonne appears, it would be different than the original. Reynold ironically would doubt the legitimacy of his own sister and would take Penelope's side. Vinter is the neutral type although in the original he was the first one to condemn Penelope but this time, that's not the case. Derrick is indenial that's why he'll take Yvonne's side. He'll be manipulated by her along with Eclis although Derrick would be much bearable than Eclis later since Eclis have a twisted affection towards Penelope. On the other hand, Derrick just find his attraction towards Penelope as disgusting. Not because he views Penelope as disgusting but himself. He thinks his attraction towards her is not right because penelope is her adopted sister so he views it as incest that's why he tried so hard to distance himself leaving him vulnerable to Yvonne's magic which is something like manipulation. Eclipse was also manipulated in the same way because his bond towards Penelope is not as strong as Callisto's and Reynold's. Vinter was not manipulated because he's a magician. He knows magic when he see one.
PS. Yvonne is the true child of the duke but the soul who is residing inside her body is not the real Yvonne but some evil entity. The real Yvonne was kind but she died already.
I read the novel last year and I tried hard to explain it. Hope it helps. Endgame is Callisto, btw.