What you haven't told us, as far as I can see, is how your husband reacted when you said you had had enough.
Did he understand that you really mean that you cannot manage these so-called holidays any more?
Will he be just as relieved as you, if you both put your foot down and tell his parents that you cannot continue to have them coming for the only ten days holiday you have?
Or will he be annoyed with you?
If you and he are in agreement that enough is enough and you have both done your duty by his parents for 18 years, then the matter is fairly plain sailing.
Start by checking with your SIL that she has invited them for Christmas, and whether she is expecting them to stay at your place? Say they cannot - you don't need to give a reason, but if she asks for one, say you are exhausted waiting on them hand and foot during your holiday, and that you really need a break at Christmas.
Why should your parents in law expect to stay with you, if they are spending Christmas with her?
Then sit down and work out where you and your husband want to go next year for your holiday, and book it NOW!
Then tell his parents that you are happy to hear that your SIL has invited them for Christmas, but that they cannot stay with you at Christmas.
If your SIL can't put them up, then she or they will need to find accomodation for them, but not at your place.
If on the other hand your husband thought you were only giving vent to a passing bad mood, you need to start by convincing him that his parents are the outside of enough and that you neither can nor will have them staying next holiday, or at Christmas.
This can be difficult and disagreeable if you are not in agreement regarding his parents.
Now is also the time to tell him frankly that you are afraid that you and he will end up talking to each other as his parents do, and that you just cannot bear the thought. And stop him every time his reactions resemble theirs.
He needs to know that you find it hard to cope with and that you will not put up with manners like these.
I am guessing you have put up with far too much from your parents-in-law over the years, so changing things will be hard, but there must be changes, as you can no longer cope, or should be expected to.