Although there may be many benefits to taking your in-laws on vacation with you, there are also some not-so-positive things to consider. For example, figuring out the financial responsibilities can be stressful and possibly bring about some tension on your vacation. Deciding who should pay for what and keeping up with financial expectations can really put a damper on your vacation if you aren't careful and don't plan ahead.
Also, some people just have different vacationing styles. While you and your spouse might want to relax on the beach and take it easy, your in-laws might want to be pretty active (or vice versa). If there are expectations for you and your spouse to spend most of your time with your in-laws, you could get sucked in to doing things that you might not ordinarily want to do on your vacation.
Lack of privacy is also a common issue when vacationing with your in-laws (and really, just about anyone). You and your spouse will likely want time alone and you should definitely have your own bedroom to keep from feeling crowded by your company. If you just can't get away physically, try to mentally divert yourself by reading a book or listening to music (Source: Voo).
In some cases, vacationing with in-laws just isn't a good idea if there are any unresolved issues or arguments. It is likely that those things will come up on your trip and will be a surefire way to ruin your vacation.
If you must vacation with your in-laws against your will, handle any issues beforehand and plan ahead to make certain that your trip goes as smooth as possible. After all, a vacation is supposed to be fun and relaxing, so do your best to ensure that it is.