Oh Gosh...
Do you think it has something to do in the choice of your fiance ?

Or maybe she didn't get to have the wedding she wanted ? I believe that our mothers/MIL act or involve themselves like their own mothers/MIL did.
I'm not in psychology and so on, but I think that somewhere, a wedding, a birth, a death is a family reunion that reminds for some difficult stuff, and for others nice stuff... and that they do not really know how to deal with it...