My dad's mum brought 'her proper' stuffing to christmas every time she came to our house for christmas lunch as she was so horrified by my mother's stuffing - which is her family's traditional stuffing.
She would then get upset when there was lots of her stuffing (sausage meat based, pretty horrid) left but the rest of us were all fighting over my mum's stuffing (fresh breadcrumbs, fresh thyme, melted butter and sprinkle of salt, some cooked in the bird, some in a separate dish as there was never enough) - such a simple recipe but completely and utterly gorgeous.
which kind of served her right for bringing it, she got to eat what she liked so was happy in one respect, but then got upset because she had been upstaged by my mum's better cooking!
However it had been going on for so long it had turned into a family joke that she would always turn up with her stuffing so it had got beyond the being rude stage, was more like batty old person. 
Definitely think it is rude of your PIL to insist on you doing things their way - either they do things they way that their host does (wonder what their reaction would be if you asked them to do Christmas your traditional way when you are next a guest there...)
if they persist about bringing stuff with them, point out there is no room to cook it or warm it and you have got enough food for the meal, it is all sorted thank you very much.
And if they actually turn up with anything (maybe let one dish of stuffing in if you are feeling charitable) on the day, I'd take it off them very briskly, say 'thank you, but I told you that I HAVE GOT CHIRSTMAS LUNCH SORTED, we won't be needing this, you can take it home with you and have it for boxing day' and take it off them and hide it where they won't find it until you give it back to them to take home. Disappear off with it and mutter loudly to your DH so they don't hear actually so they damn well do about how rude they are to have brought stuff when you asked them not to, how they have ruined christmas and how upset they are about how they have dismissed you and your traditions and how you have been polite about their wretched OTT traditions all this time.