Several possibilities here, and let's start with the least "worrisome".
The first possibility is "product". Both of my sons had sensitive skin, but my second is especially sensitive and prone to diaper rash problems. We use almost exclusively products without added fragances or chemicals for him. The littlest things can make a difference. Even aloe (which is supposed to be soothing, right?) irritates his skin, so it's unscented, fragrance-free, gentle care baby wipes. Texture can make a difference even among these. Huggies seem to work well for him. We have had less success with others (Costco and Target's brands). Diaper rash cremes can also make a difference. The creamy Desitin type is not so good. Petroleum based is better, the basic A&D seems to do the job well. Bottom line (no pun intended) is that if your in-laws are using different products than you use at home, it could be causing the problem.
Second possibility, diet. Some foods are more irritating to the intestinal tract than others, and may cause his stool to have a higher level of acidity. So, if either you or your in-laws is feeding him spicy foods, or allowing him to eat a lot of raisins/applesauce/citrus fruits, if he is processing that food at their house, it could irritate his bottom.
Both of these are worth thinking about and easy to work with them to resolve. On the downside, if these are not the problem, and it is "something they are doing," it is more likely that they are not changing him frequently enough and that he is sitting in dirty diapers too long, than that their wiping technique is the problem. Obviously, this is something that you would have to address with them too.
I would suggest approaching the whole issue as something that you need their help with. Tell them you need their assistance to figure out what is causing his skin irritation, so you can eliminate the problem and he won't suffer...
Hope this helps. Best of luck.