I understand where Cecil is coming from but at the same time I think Jax is kind of a victim because their dad (the king) was already sickly bedridden, meaning that he would die any moment. After the death of the king, who would claim the throne? Naturally one of the princes, however, they were young, so it would lead to the uncle somehow working dirty behind the scenes and say that they are too young and will reign till the princes are ready. Which is what is happening currently. Therefore, I think the uncle is threatening Jax about hurting the brothers, for example poisoning Darcy. Most likely the whole thing was the uncle's plan, but Jax takes the blame and PRETEND to be an ally until the right moment to finish off the uncle. This is explained in the previous chapter about the chess game and Sophia's analysis.
Looking forward to Sophia's revenge and claiming back her title as a princess/heir to throne.