[He hesitates in the pause Grace gives him, because holding onto this last thread, Lan Xichen is afraid that he'll no longer be welcome if she becomes angry as well. There is no real reason to assume that she would turn him out, but the instinct of dread still lingers as he clings to the calm she always offers him.]

I... I'm afraid it is difficult to explain. Putting it too simply would be unkind, but I do not wish to defend any wrongdoing either.

[He looks up with a smile that threatens to shatter.]

My brother is angry with me. [Lets start there.] I have not angered him in, I believe, all our lives. I feel I have betrayed him and myself.