It's been a long time since I saw or even listened to Die Fledermaus, but I've always enjoyed it when it comes around. Last night I heard the first two acts from the Met, then had to go over (at 9pm British time) to the TV to keep up with the 3rd and 4th episodes of the second series of The Bridge.
Anyway, I seem to recall that when I've heard Fledermaus in English it's never been "Brotherlein" and "Sisterlein" but "Brother mine" and "Sister mine".
I thought the Orlofsky wasn't too bad (for a counter-tenor). Nice to hear Christopher Maltman, though his English accent sounded rather odd when speaking to Americans. Some of the jokes fell pretty flat, too. I was a bit dubious about the overture (rather heavy), but my ancient Clemens Krauss recording is still my favourite, though it doesn't have any spoken dialogue.