Kimmerz, I understand the "phase" your oldest daughter is going through. He knows that your oldest daughter sees right through him and that's why it's more difficult for them to commuicate w/each other. He knows he can't fool her. On the other hand, mlcers tend to be best buds w/the child that is the easiest to get along with. The one that will not question their every move and also will open up and tell them what is going on in the lbs' home.
I do understand the frustration here and I have heard the same thing from other parents about how the mlcer singles out one child as the best bud.
Suggest that your oldest D observe as her father and her sister interact. It might give her some insight as to how to draw him out. If she wants to know something, tell her not to wait on him, but to ask him directly. Eventually he will learn to tell her, but it has to be a practice that she continues to use and not drop by the wayside.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.