Reading what you just wrote, Bert, I can't help but remember that it is mentioned that she has (at least) two sons. No daughters are mentioned. Which makes me wonder if part of this is because she doesn't have any daughters, and she also has very strong ideas about how she wants her son's wedding to be, so she's inserted herself.
Impossible to know, without going into any detail.
Somewhere on the internet, I remember a forum thread that I read recently. It was about boundaries and control, and MIL problems. I remember someone saying something along these lines "I feel bad for women of a certain generation. When they were brides, it was the custom for their mothers/families to plan the weddings of the children. And then the world changed, and that became controlling, so by the time their daughters grew up to be married, the norm was for the daughters to plan their own weddings. So this one generation of women never got to plan a wedding, and maybe that causes them to lash out?".
I don't know if that's a pan-American thing or more often happened in certain parts of the country or specific classes or what, but I can definitely say that my mom was part of that generation. There's a lot of things she didn't get to do because when she was younger, her parents made ALL the decisions, just like most of the parents of her cohort.
Anyways, while we're doing lots of rampant speculating, I wonder if that is coming into play here, or if it comes into play very often still, in regards to wedding planning.