Have Realistic Expectations
It is important to have realistic expectations. If things have always been tense with the in-laws, it is unlikely that this time will be any different. Don't go into each interaction with the notion that things will be completely different. This will only lead to disappointment, like every other time. Instead, try to take into consideration that it takes two sides to argue. If one person simply walks away there is no argument to be had. It won't be this easy with all in-laws, but the phrase, "I don't want to argue about this," is a very good start to making trips more bearable.
Don't Instigate Arguments
As easy as it might to be walk away from an argument started by someone else, it can be just as easy to instigate a discussion in which no one wants to back down and that is when feelings will get hurt. When someone makes a point that might not be agreeable, think before speaking and avoid aggravating the conversation. It is simple to start an argument, but more difficult to stop one before it even begins. This may take a bit of practice, but focus on not being an instigator for the sake of peace in the household. Everyone will appreciate this and others might follow suit, leading to future visits being much more tolerable. It can have a domino effect in a very positive way.
Remember to Keep Calm
Finally, remember to stay calm and remember this trip will not last forever, no matter if it seems that way. When things seem too tough and tempers run high, take a moment to remember that the situation will end soon. Do not let the situation cause more stress than necessary. The in-laws in question might harp on the same things and say negative things, but they can never control how much it affects someone. Put a stop to how much it hurts by shrugging off snide or critical remarks. Keeping calm and not letting it be a big deal can lead to less of the hurtful behavior from the in-laws. Don't sweat the small stuff in this situation and it can lead to a much happier weekend.
Take control of the situations that cause problems and refuse to let them interfere with the weekend. Learn to shrug off snide remarks, don't instigate arguments and always remember to have realistic expectations.