I can think of nothing so pertinent as animals' instinctual marking, but these people know what they are doing, because they are human beings who are not alien to the cultural norms of human toileting, and worldwide, leaving a mess in the toilet is an act of disrespect and contempt.

I like the script for tearing the band-aid off and bluntly calling the fouling what it is. Send it as a group email or texts to all of the offenders together. Show them they are seen.

If the husband continues to neither back her up nor clean up after his family's visits, she should reasses her relations with his family and him.