I recommend you have a conversation with your in-laws. Explain to them that you do not like visiting them as it takes you away from your gaming addiction.
Ahh, you sadly make a lot of assumptions that do you no justice. When my in-laws are around and not out doing the farm work, we generally do have very nice conversations. Yes, there are moments when I do not agree with their culture and way-of-life and they do not appreciate mine. The real issue is when they're not home, I generally have very little to occupy my time. This is where my gaming addiction (that's one assumption you made that I will shamefully admit) comes in handy, allowing me to pass the time while I wait for their return.
You may ask why don't I go out and help them. Well, I'm not familiar enough with riding horses to be useful with the cattle. I cannot run the heavy equipment they use to harvest their grain. Really, I am simply the chauffeur for my family so they can visit with Mom and Dad/Grandma and Grandpa. Once I'm there, I'm as useful as a bicycle to a fish.
It is really surprising that Blizzard haven't figured out a way for you to be able to use their game to properly get your feelings across to your relatives.
Perhaps they can provide a farm equipment simulator so that I can be more useful to my in-laws. I probably wouldn't buy it anyway, as I don't usually play the sim games. Besides, learning to drive a tractor via a sim game is much like learning to be a rock star by playing Guitar Hero. No one would take me seriously.
Perhaps you should consider why it is you must fill every waking moment with something that takes you away from human contact.
Perhaps because sometimes that human contact is not present, or because my very nature is to be socially inept and I have a very hard time overcoming that nature. As weak as they are, I have my excuses and I plan to stick to them.
Thankfully, though, Diablo III isn't the only game in the world. You should be fine playing it at home and finding an alternative method of shunning your in-laws with. I suggest you get a game you can play on your iPhone/Android device. This way you can sit right infront of them while playing, so your disdain for them can be made all the more obvious.
Again, you assume I find my in-law reprehensible. My level of respect for them exceed most people as they spend most of their lives out in the fields tending to their livelihood, plus they can serve a family meal that would feed a small nation. I would never avoid social contact during one of their meals. When I am around them we tend to argue politics and culture, but I understand them and they tolerate me. At times I need to step away from a conflict and cool down, which is one of the reasons I have Plants vs. Zombies on my tablet. Great diversionary tactic.
I'm sure you have a wonderful relationship with your in-laws. I just find you're overly defensive attitude a poor quality in a person.