Secondly, recognize you aren't going to change the in-laws but can only modify your own responses. Think of trying to lift the Washington Monument. That's about your chances of success of changing a whole family system. Now think about having an attitude that you had to do it because your happiness depends on it, or else your life would totally stink. That would be then your perfect recipe for depression. Forget that! Don't get sucked into extreme thinking. But realize you have infinitely more leverage when you concentrate on your own responses. Think of it as being a scientist, experimenting with one piece of input at a time. When you're not sucked into the fight, other ideas will come to mind. Sometimes your best and most creative responses will come to you from your own deep inner wisdom when you least expect it. Do this exercise. Think of a model person, not yourself, handling a family like this in a specific scenario. What would the model's ideal feelings be, their thinking be, their actual behaviors be, even under the most trying circumstances? List the ideal nouns and adjectives under each of those three categories--feelings, thinking, behavior. Play it out in your mind like a movie. See the model handling it maybe not perfectly, but in a way she or he is satisfied with, and exits with their dignity, inner calm and peace intact. Then replay the movie with you in the star role. Try to get the specifics of it-- sounds, sights, textures, even smells; see and hear what is actually said and done as you handle it in your best fashion. Breathe deeply while you close your eyes and run the movie one last time, and it will anchor in you for future use.
Lastly, practice in real time. Don't avoid your in-laws, but use every family interaction as opportunity to grow your vision of your partner as the unique and developing person he or she is. Use each visit to practice being the unique couple you are, each phone call to practice maintaining your cool, each exchange to practice your greater repertoire of skills even in the middle of high family drama. This is hard stuff and everyone deals with setbacks. Be satisfied with days when you just break even and only manage to not slip toward the zero end of the scale yourself. Relax, breathe, and give yourself credit for making the earth a little better place, knowing that you are getting better and better at not duplicating the rigidity and reactivity of these families but modeling a higher level of being.