1. Remember that you and your partner are now the primary commitment you have. Choose your partner over your in-laws (I recommend choosing your partner every day and letting him or her know that). This means if there is a question, issue, or dispute, you side with your partner in the moment. You can talk about it in private later.
2. Use the time with your in-laws to just notice family patterns. I am not encouraging you to talk about it right now. And when you do, ask the person whose parents these are what he or she noticed before you start talking!
Notice such things as: Who talks? Who is quiet? What topics are discussed, what topics not broached? Are the words and body language saying the same thing? Who drinks, and how much? Is that a concern? Do you or your partner revert to childhood behavior around your in-laws? If so, what brings it out? What else do you notice? Do you see, hear or feel interactions that are familiar in your relationship?
3. Take breaks. If you have kids, leave them with your in-laws and go out on a date (in which you don't talk about your kids or in-laws). If you don't have kids, steer yourselves (or your in-laws) out on an adventure (a walk, shopping, takeaway meal, etc.).
After they go home, relax. After you relax, you can talk about the visit. Ask questions and be curious about what the adult child noticed about his or her family. When you share what you noticed (after listening well), you might phrase it, "Did you notice . . .?" This may make it easier for your partner to take in what you noticed without becoming defensive.
Your in-laws are unlikely to change. Disappointing, I know, but you have to deal with "what is." Therefore you and your partner need to figure out together how to handle in-law issues.
Good luck and happy holidays!