Now, my in-laws love to bring over their dogs. (Boarding dogs is expensive, I am keenly aware!) My mother-in-law brings her two middle-aged dogs. These dogs are always filthy - the stench is bad enough that she'll comment on it. Nonetheless, they promptly sit on the furniture, climb into the guest bed (staining the sheets and both sides of the duvet with mud, eye weepage, food particles), and bark uncontrollably! My MIL sees nothing wrong with any of it, as this is the behavior that is allowed at her house.
At the same time, my sister-in-law brings her young adult dog (1.5 years old). It bites at our dogs' ears/eyes/nose/tail when playing together and unceremoniously destroys our dogs' toys when playing solo. Despite my requests otherwise, SIL feeds/waters her dog on the hardwood - and the dog spills food and water everywhere without cleaning up. But perhaps most importantly, it takes at least one (more often two) big, steaming dumps inside our house every time she visits. She never reprimands the dog for it, as this all happens regularly at her place.
I've tried hard not to display my absolute disgust, but insist on all three guest pets being crated if the humans go out unaccompanied. My in-laws are offended that I ask them to crate them. Alternatively, I'm offended by their dogs' behavior and wouldn't dream of bringing my dogs to someone else's home if they acted that way.
Can I impose my house/dog rules on someone else's animal(s)? And if so, how can I do so without starting a Cold War?
Disgusted Dog (and House!) Lover
Your in-laws are family but they are still guests in your home and you are right to call them out for their behavior. Their dogs are also guests and, yes, you can impose your rules on them. Chewing up your dogs' toys seems possibly inevitable (isn't that what dog toys are for?), but pooping on your floor and soiling your bedding are insupportable (and, also, gross).
The second part of your question is how to impose the household canine rules without creating a family feud. You didn't mention your husband and whether he is on board with laying down the law, but he should be. Ideally he would be the one to firmly communicate the expectations to his mom and sister. Families squabble about frivolous things all the time. Showing respect for your home isn't a frivolous thing and, to my mind, is worth a squabble or two. Be clear with your in-laws about what is okay and not okay with the dogs so they can't plead ignorance. If they aren't willing to go along with your wishes, perhaps they'll all have to find a different doghouse to sleep in when they visit.