Hmmm, it's never been *offically* announced that they are separated, even though it is quite obvious to us spectators. They've been living apart since August or September of 2009, and of course Ernst has been flagrantly flaunting his peccadillos for the whole entire world to see!
But, imho this estrangement does not extend to his children, whom are still close to Caroline's. In fact I've seen photos of Pierre visiting Ernst Jr. in the Middle East just last year (I'm sure if you wanted to do a search on here you could find the aforementioned pics). Also, I recall Andrea went to school/university in NY with one or both of them. And I've seen pics of the whole bunch together (i.e. Ernst Jr., Andrea, Christian, Charlotte, Pierre) walking along at some event in casual clothes, but I admit that was taken a long time ago.
Still, I maintain they are close, they've grown up together for more than half their lives now, since Ernst was 13, Andrea 12, Christian 11, Charlotte 10, and Pierre 9. Additionally, they're all connected through Alexandra, their sister on both sides.
So here's hoping they do show up at the wedding!