Spoiler

Venessa(jona's birth mother) was a princess who was kept from being able to do anything because of her father. Due to this, Venessa entered a rebellious age and married the duke to run away from her father. She used her maid to struck the deal with him because she knew the duke loved Sophie. She wanted freedom from her father and he wanted Rosie. The railroad deal was something the former king made, because he doesn't trust the duke and thought the duke just wanted to steal Venessa from him for power and then toss her aside later. Basically the former king made the deal thinking it would keep the duke from tossing Venessa. He didn't know that the duke doesn't care for the railroad deal.

Venessa on the other hand, is a promiscuous princess. She slept around, so when she got pregnant, she doesn't even know who the father is. It was unfortunate she died when she gave birth to Jonas, so her freedom was short lived. The duke felt some guilt over not being able to kept his end of the bargain so he just accepted and take care of her son for her. It didn't matter to the duke but he knew Sophie would care, so he kept it a secret and pretended the child was his. He didn't care if the heir is Rudy or Jonas, or even Luca.

The duke said since Luca used the cup, he should pay back the price and stay heir, just because of Sophie. The cup can only be used once. The goblet that was passed down by the Winterwald family. In fact only someone with royal blood can use the goblet, so no Winterwald would've actually ever been able to unleash its magic. The reason why Winterwald has the cup is because of the greed from the children of the founding King. Winterwald was entrusted to hold the cup to prevent any royalty from using their greed to make some crazy wish with it. The secret was removed from the royal family by the founding king but Winterwald continues to hold the secret and the goblet. Winterwald was tasked to keep the royal family from accessing the cup.

Now going back to what was previously mentioned, the goblet is passed down to the heirs of Winterwald and only someone of royal blood can use the cup. Judith recalled Malinda and Franz saying Jonas was killed by the duke. She started to wonder if the duke would've killed Luca as well if it was Luca accessing the cup.

The duke admit to killing Jonas. Again as mentioned, he didn't care who his heir is, and in fact he didn't care who can make a wish or not, but what made him care was the words Jonas said. Jonas wanted to be king. He was arrogant, greedy and had a horrible personality. The fact he wanted to be king would be called treason if he was to be heir of Winterwald. But what really made the duke upset was Jonas looked down on Sophie, who was his mother's maid. Jonas think Sophie is an eye sore and wanted to kill her after he becomes king. The duke knew Jonas could use the cup to become king and try to ruin the country and kill Sophie, so he killed Jonas. Of course he kept all that a secret from Sophie since he thought it would hurt her.

He asked Luca what happened to Sophie in past life. She died of anxiety after Rudy had been killed by assassins. The duke didn't ask about himself since he knew he must've let go of everything after her death. Rudy was the one who looked confused. He didn't think his death would've effected Sophie. The duke finally agreed to withdraw Luca from the family registry, but Judith have to keep all this a secret from Sophie and particularly what Jonas thought of her. It was obvious for Judith to do so anyway.