Personally, I would view it as less for me to deal with. If MIL wants to limit her discussions to her son and whoever else…fine by me. I wouldn’t care much at all…its not like there isn’t enough to deal with on a daily basis. What if they start asking for opinions and then later blaming you or worse you get accused to interfering. These situations have a way of messing up. So, I’d rather not be a part of them.
She shouldn’t complain. Count her blessings that she does not have to live with them and ignore the rest.