Elijah knew that after his brief yet informative meeting with Niklaus and Freya whom he now truly believed to be his elder sister. But after what she had done to Katerina he could never forgive her for that and she knew that. But what she had said had him convinced that they were going to need her on their side if they had any chance of defeating their aunt Dahlia.

Entering the parlour in which he found Katerina sitting engulfed in the book which he had given her about Nadia Petrova and her life until 1986. Leaning against the door frame he knocked on the side making his presence known. 

If this was going to go ahead then he needed to be the one to explain what was happening "...Forgive the intrusion"