Oh, I was thinking of the mistresses due to marry Firefighter Absolut and Station Officer Strongbow, Emily!
I've just finished the Müller Twins fill-in. I very much enjoyed it and would totally recommend - it's well written, nice and pacey and EBDish. But. BUT. I'm actually slightly surprised GGB OK'd it. I thought they were supposed to be very constrained by 'not too much can happen, because if Big Things happen, the books would have been constantly referring back to it' thing, and to me the central characters violate this idea in a pretty big way.
The twins' appearance is explained by a throwaway line in Exile where the staff are commenting on the German girls they can't expect to bring with the school to England - Alixe, the Linders, and these mysteriously never-mentioned Müller twins. Nice idea. But the fill-in is set between Jo Returns and New and of course the twins don't get a mention at all in New. Later in the series, that would be totally normal - as per recent comments on this thread about Prunella - but I don't think it would have happened at that point. Also, there's a big thing about them being "the school's first twins" and I think it's unimaginable that they wouldn't therefore have been recalled as such in every bloody subsequent book at least on particular occasions, such as the arrival of the Highland Twins, Dawbarns, possibly the various births of Maynard twins etc.
I really have to emphasise that I don't particularly think this detracts from it being a v enjoyable read - but it did surprise me, for all I've read about the difficulties of creating GGB fill-ins for that reason.
Probably more subjective: I actually also found the title characters rather unengaging because they feel so caricatured as 'good twin' and 'bad twin', pre-reformation. I can't exactly call it un-EBDish because there are a number of occasions of EBD herself doing precisely that (in spite of having Matey criticise Jo's first draft for it!) - Eustacia is the obvious example that springs to mind, or indeed Len. But on those occasions, I always feel that EBD is being somehow un-EBDish, and I wish she wouldn't, and I do feel similarly about this.
It is a great book, though. Would recommend. And it is so, so nice to abandon Feud to return to late Tyrol... For me, the very best period is the run of books from And Jo to Gay from China, so smack-bang in the middle of those is a very nice place to revisit. :)