Here's what we have from the books on Michael:
"Wee Michael's just married, but his wife's breeding." — Jamie to Claire in Voyager (1766)
"On a happier note, we have had word from Michael in Paris; he continues to prosper in the wine business, and thinks of marrying soon." — Letter from Ian to Jamie in Drums of Autumn (1767, about 6-8 months after Claire's return)
In ECHO and SPACE, we experience Michael's grief upon the death of his wife, Lillie, and their unborn child. (1778)
If we take all of that as fact (and not error), then it looks like perhaps Michael had a first wife who died, then met Lillie while working in Paris with Jared. Still, if Lillie was his second wife, one would think that the fact that he's lost two wives (and children??) would have come up in ECHO or SPACE, but that's just me. The outlier is the first one in Voyager, and we could potentially brush that off as Jamie losing track of his relatives, who's married to whom, who's having kids – but Jamie is usually freakishly on top of all that sort of thing.