I believe the leading cause of holiday stress has nothing to do with how you actually feel about your family. You can take all the pop psychology theories that Oprah & Dr. Phil spew and throw them right out the window. The true underlying cause of stress, frustration and all the crap that happens when families get together is, well, literally crap. More specifically, where we crap.
The one thing I miss most when I travel is my own bathroom. It's like my own little Fortress of Solitude. I know where the extra toilet paper is if the roll is empty. I know that when I'm in there, no one's gonna bother me. I know that with the fan going whatever is happening can't be heard by anyone else. I'm safe, secure and relaxed. And, as we all know, relaxation is very important when taking care of business.
But when we travel, everything that was certain becomes an X-factor: Where's the backup TP? No fan...can anyone hear me & how much Glade do I use? Why are people standing outside the door, yelling up the hall about where we're going to dinner? Do they not know what's going on in here? Let's not forget the horror of discovering that maybe your family only uses Ply when you're used to, at the bare minimum, 2 Ply or quilted. That throws off your sense of proportion and can lead to the 5 words you never want to say to your host, "Do you have a plunger?"
All of this uncertainty leads to tension, which of course causes people to get off schedule. Being off schedule for a day is no big deal. But if your visit lasts, say a week to ten days and add in multiple locations, a wide variety of food choices and having to wait on other people to finish their business before you can start yours and folks are going to start getting a little cranky.
This conundrum works both ways. It's not just when you go visit relatives. It happens to them when they come to visit you. If Dad or Cousin Bo seem a little edgy by the end of the weekend, it's not because they don't enjoy your company...it's because they're backed up.
Also, if you're having guests, please make sure everything works and all needed materials can be easily found. Don't hide the spare rolls in cute, discreet boxes or cozies. Your guests need visual confirmation in an emergency that everything they need will be literally at arms length. Having to yell plaintively "Need a little help" is a humiliation that should be avoided at all costs.
Unfortunately, there is no easy solution. Unless you want to run to the nearest convenience store when nature calls (and public restrooms are potential minefield in their own right), you're just going to have to deal with it. So, I encourage patience and consideration when you've got to be away from your own facilities. Remember, we're all in this together (though not literally, because that would be gross.)