They've always had a close bond and are very protective of each other, but after the Calamity and their father's passing they became a bit more distant as both tried to recover on their own ways. Caius's travels and WoL business also keep him away a lot so he can't visit often.

He went back home post-EW to try to recover, that's when Celia in turn took care of him as he always did for her in their childhood and they had all the time in the world to talk. He still tends to forget she's not a little kid anymore and is in fact quite capable, not needing anyone to guard her.