I'd originally thought that she was doing so to cool down a relationship that Mike is quite frankly too young for but we all know that it takes a year and a half from now for Elly to learn that Martha exists and that Mike's been in an on-and-off thing with her all that time. What's happening is pretty much her trying to make things better by being with family and maybe showing Mike and Lizzie the real meaning of Christmas: being harangued about being spoiled and lazy by arrogant and clannish hayseeds.
We start things off with this being discussed behind Michael's back because he might object to any plan to make him Suddenly Wonderful by exposing him to the beauty of an ice-cold winter in beautiful Middle Of God-Damned Nowhere, Manitoba because central heating and pipes that aren't frozen solid are a luxury that's making him bad and cruel and mother-hating. So, it would seem, is not especially liking old people who want to doze away the day so KEEP IT QUIET, being reminded that a Grunt-Burg used to be a cow, being packed in like sardines and, yes, being lectured to be smug fist-magnet farm-folk about how foolish he is for being materialistic and rebellious and the like.
What caps things off is that when someone points out the defects in Elly's logic and how the results are that Mike feels miserable (and was forced to stand out in the cold to Think About Being A Bad Guest by his graceless, thin-skinned moron hosts because he finally told that snippy, yapping annoyance Laura where she can shove the attitude she needs to lose yesterday), she gets all angry about how wonderful her ill-thought out plan is. The only thing that would have saved this is if Lynn had followed through on her plan to pack Danny and the gang down to Milboring so that nitwit Laura can get cut down to size.