So the Duke cheating on his wife with Camilla's mom... I'm so disappointed

i don't think he cheated. i think all three of them have diff mam's

Technically he did not. He has amnesia while he's with Camilla's mom. Then suddenly regained his old memories and lost his memory when he was with Camilla's mom.
That is why he thought he lost the red ring (Camilla's ring from her mom) and didn't know that he gave it to Camilla's mom, since he lost his memory of that period/timeline.
He regained his memory before Camilla's mom noticed that she was pregnant with Camilla.

lol it's not like he deliberately cheated on his wife, pretty sure it was mentioned he had amnesia when he met camilla's mother

I don't think so. I can't remember the timeline of how the parents (all of them coz wtf are they doing) just friggin planted seeds but his eldest is adopted. Then yeaaars later lil bro is born. I don't think he was married yet when he met Camilla's mom. I honestly don't remember those finer details tho

Oh! I don't remember the amnesia thing, I need to read this arc again I guess

I don't remember the amnesia thing at all, I don't know if I skipped a chapter or something

It's the appearance of the eldest that confused me, but apparently there amnesia in the mix so I gotta read this arc again