Deny him.
Julia knew he could not lie so he decided to blend the truth. "I knew about father's plans but it was only after you left. I was going to tell you about it but I was scared my brother will have his eyes on you and realize I am still alive," Julia said.
Although his statement was logical, lady Catherine felt he was lying. She believes that the only reason why he did not tell her was that he did not trust her or he just wanted to manipulate her to seek revenge for him.
Her late father(general Thompson) had told her countless times that the father and son acted the same.
First king Roderick had used her father to kill his brother and plant the evidence on Ericsson. If there was no third party then nothing would have ever linked him to the death of his brother.
More so, Julia himself had told her that the only reason he approached Ellen was to use her against Sébastien.