I'm no expert at dog psychology (we have trouble getting our beagle not to mark), but it might be worth considering the human psychology. You've mentioned that you feel your SIL and brother are being selfish, and spoiling the dog (and their kids). You indicated that you yourself would be horrified you had a dog and it did that at someone else's home. So, when you contemplate the dog's pee, as you cleaned it up, you had at least TWO things going on. First was just the minor nastiness of having to clean up the pee. The other thing going on was that it galled you that you should even HAVE to do it.
It seems like you're hoping that your brother and especially SIL will come around and realize that they're being selfish and inconveniencing other people, and that they should take better measures to train their dog and deal with the pee issue in whatever way works.
OK, that MAY happen. Chances are, though, that it won't. You're hoping they will change. A large part of your annoyance is probably due to the fact that they haven't changed.
I would suggest that you push for a "no dogs" approach to the upcoming trip. Just explain very simply that their dog peed all over things when they were over at your place, and you're just not OK with that. And yes, you get to be the bad guy. Just own it and try not to let it bother you.
At some point during the trip, your SIL (if she comes) will probably pontificate about how much better it would be if SF* were there too, and how much money they're having to spend on boarding or whatever. Just get on your happy face, and say, "yes, that's too bad. It's too bad your dog pees all over the place." Don't argue, don't expect anyone to change, keep it low key.
That's my 2 cents, FWIW.
*Special Fido