Ok here's a fun one for everyone:
My fiance's sister-in-law's parents separated three years ago and are in the process of getting a divorce. They are a very traditional family, and at most family occasions they seem to be attending together and sort of "keeping up appearances." (Background: they had been unhappy for 10+ years but stayed married until their children were grown.) Last year they attended their daughter's (my FSIL's) wedding, as though they were still married. My inference is that she has issues with the separation.
The reality is that they are going through a divorce and by the time my wedding happens, it will have been at least four years from their initial separation. My knowledge (and personal standard) of etiquette is that all non-married guests are invited with a +1, however, based on the way they historically handle these types of family functions, I am positive that they will be likely to continue the game. When they met my parents at an event a couple of years ago, they chatted all evening, without once indicating that they were separated- they acted like it hadn't happened and my mom was shocked when I told her they were getting divorced.
Am I excused from giving them each a +1 in this case? It goes against everything I know, but I feel like this is such a different case, and I have a very strong feeling that sending their invites WITH a +1 will feel like a slap in the face to my FSIL and sort of open the door to what they seem to feel hides the "skeletons" in their closet.
The other (smaller) side is that I don't know them very well, and don't want to end up paying for 4 guests, were they to bring a sibling or friend (I don't think there is the slightest chance that one of them will bring a date..but I guess you never know.)