The firstly sounds extremely odd to me. In AusE, we would say, "One of their wives", or "One of the wives".
PS: Sorry for the 'Dislike" Grumpy. I'd prefer a 'Disagree' button. Which sociolect in UK says such a strange thing?
That's quite all right, Raymott; no offence taken at all.
I really couldn't specify a particular British sociolect which says such a thing [nb "says"; no-one would write it], but I would confidently assert that its use is pretty widespread in casual day to day conversation throughout UK society. Deep down, we all know that it's indefensible grammatically, but it does the job very neatly. After all, it avoids the implication that one or more of the men has several wives ["One of their wives..."], and it trips off the tongue more fluidly than "The/A wife of one of them....".
Talking of things that sound odd, I have never seen or heard "firstly" used as you have in this post. Is that common in AusE?