Go meet your girlfriend’s parents. You will live, and she will find you a better listener when she has some sense that you know who she is talking about.
Yes, you will be the zoo animal for the week or the weekend. This is a sitcom that doesn’t have new characters strolling through all that often. You will survive it. Consider it time off of Purgatory for the sins you have committed by staying in your comfort zone instead of doing what charity demanded. Taken in that way, you will profit from it.
Yes, you could conceivably wind up doing some sort of intimate care for this couple some day, if it comes to that, but it probably won’t. If it does, it will be much worse for them than for you, so let them worry about it. An invalid can be grouchy, but there is a real limit to how much “controlling” they can do. That’s probably the most difficult part of old age for the very, um, opinionated. In a large family, these duties are spread around so much that often the help of the in-laws is only required in a support sense. Look at the bright side: Even if you offered, your future MIL would not let you take care of her. You’re probably off the hook on that one!
Avoid rash judgment, and take everyone’s attitudes towards everyone else with a grain of salt. In a large family (and I come from one), everyone has their own mythology going. More often than not, you would not recognize one set of family members by the description you got from the others. It’s usually errs that much on the side of rosy, bleak, or both. Mythologies, even private mythologies, lend themselves towards making each character into an archetype, described by a limited number of strong adjectives. Nobody is lying, but no one has such a clear view that they’re telling the whole truth, either.
Be a good house guest. That will please your girlfriend. Read the chapter on charity from 1 Corinthians, and take care to lean in that direction. That will please God. If you do that, you won’t displease anyone in any way that makes an eternal difference.
Remember: This may not be your #1 choice for how to spend your vacation days, but it will not last forever. You don’t have to enjoy it. You can put a smile on and Just Do It.