It's based on her boundary violation, not my ungratefulness. The issue is more than an invasion of gifts; it's an invasion of our family and the values that we hope to instill in our children.
I get what you're saying, I really do. But I also think one needs to choose one's battles. With my first baby, I was rather precious (not saying this is you -- it was me) about him only playing with Waldorf-type stuff, non-licensed merchandise etc... etc... And I used to get mad when other people didn't get that and repeatedly gave us stuff outside the boundaries that I had set. After baby #4, I'm just happy that people think of my kids at all, and I no longer feel like I need to control how they do that. If we get stuff I really, really hate or just don't have the space for, I have no trouble getting rid of it. But I don't get all worked up about plastic (which was my big issue) anymore -- I just relegate it to the sandpit until it breaks. I guess I just feel like I have bigger fish to fry now that my little ones are getting older and exposed to all sorts of different influences and perspectives in the big, wide world. I'd get worked up if the relatives gave us racist or religiously intolerant stuff, but if they just want to ship way too much material stuff at us, then I just feel grateful for the opportunity to teach the kids about need vs. want, crap vs. quality, sharing personal wealth with charities by donating useful possessions etc.. etc... It can be a great learning opportunity, and maybe you could be grateful to your MIL for funding that IYWKIM? That might help ease some of your angst, which is ultimately hurting you more than it's hurting anyone else, and save you from having another miserable Christmas if your MIL doesn't change.