Holidays with the Inlaws!

It's always fun!
So, I just got home earlier from an evening with my wife's side of the family. It was designated as a Christmas party, and even Santa visited. Now from the heading of this paragraph you may see some implied sarcasm. Well, there is a little of that, but for the most part, I always have a decent time with Stacy's side of the family. There are a lot of people that I really enjoy talking with and hanging out with. Stacy has a bunch of cousins that are close in age to us, and they all have kids, so there is always something to talk about with them. Then of course there is my mother-in-law. 
I'm not her favorite person!
Well, let's just say that there is tension when her and I are around each other. I'm not sure why. When I first met Stacy, I was the greatest guy in the world. My mother-in-law(from here forward she will be referred to as K) thought I was just swell for coming along and making her daughter happy. Then about the time Stacy got pregnant the first time, something changed. Now, I'm a piece of crap. I've mentioned that I am experiencing some physical problems that I think is Rheumatoid Arthritis here a few times. When I was feeling the worst, I was at K's house for the day and could barely walk or move. For the next couple of weeks when Stacy would talk to K on the phone, you would think she would at least be curious about what was going on with me, but no. Not even a single question about my problem. I don't think she cares. And that's fine. For a few years now she has made little snide comments digging at Stacy and particularly me. She gives me these looks and roll her eyes at me letting me know her displeasure for me without saying anything. That's all well and good, I've gotten to a point where I don't really care about that crap anymore. She can hate my guts for all I care. But, there's one thing she does consistently that I am going to put a stop to!
Undermining my authority!
Well, K pulled her crap again tonight. We walked in to the party and Santa was in the middle of speaking with all of the children in attendance. Both of my kids were scared of Santa and didn't want to go up and see him. That's fine, but then it happened. Santa came over to both of them and gave them little gifts. Sydney had a Barbie that she was holding and wanted me to carry it around for her. I told her that I was taking the gifts and her Barbie out to our van so I didn't have to hold them all night long. Sydney got a little upset, but I told her that my decision was final, and she was fine. At that point K stepped in and said "Denny, just let her keep her doll if she wants. It's fine." HELL NO IT'S NOT FINE!!! I had told Sydney it was going to the van, and now Grandma was stepping in and making me look bad. So, I gave the doll back to Sydney for like two minutes before I took it back and went to the van. I didn't say anything to K because I didn't want to cause a scene, but I was seething. That's not all though. After Sydney ate, she wanted one of the cookies we had brought as our dish to pass. K heard me tell her that it was okay and that I would get her one. "We are not letting anyone have dessert until everyone eats dinner, Denny. She will have to wait." ARE YOU FREAKING KIDDING ME???  Does K really want to keep her granddaughter from having a cookie, that we had brought mind you, or does she just want to make me look like an idiot to my daughter? I think it's the idiot thing. I'm basically positive that it's the idiot thing. Again, I was seething. This woman stood at the sink washing plastic plates and silverware for the better part of the evening, spending ZERO time with her daughter or grandkids. The same people she has given me hell for not being able to see much. She yelled at me once because she never sees her daughter due to us moving back to my hometown. So, needless to say, I was pissed off again at a get together involving K. 
I've come to a decision!
At this point, I'm done playing nice. Stacy has a hard time saying anything to K because she's her mother. I get that. She will always be her mother, no matter what happens between Stacy and I. However, I'm past the point of pissed. I will not have that woman continue to treat me like this. EVERY time we are around her, she says something that undermines my position as authority figure to my kids. EVERY SINGLE TIME! Well, that's over. We are going to their house on the 28th of this month to celebrate Christmas. I can GUARANDAMNTEE
I'm not sure if anyone reads this blog that will pass this information on to her, but if so, please know that I'm not imagining things. Stacy sees it every time it happens. She gets just as pissed about it, but for some reason we just let it go. The time for letting this sort of stuff go is over. I can just see a time in the future when my kids are older and they see Grandma basically telling their dad that they don't have to do what he says. What does that teach my kids? That Grandma runs the show and not Dad. That Dad doesn't have the balls to stand up and say that's not the case. That is what they will learn. Not happening.
Okay, I've ranted on long enough about this. What I'd like to know is if any of you have had in-law issues and how you went about solving those issues. Please feel free to give advise and tips on how I should handle the impending storm.
Once again, have a great day and enjoy your Sunday.