Talamarie, I think I remember Debbie and Dave, and whoa, you're right, they were awful.
I grew up in a resort area and my parents had a big house (not fancy, but they built an addition to it to accommodate all the kids they had.

) So we had a never-ending stream of visitors. The general MO among one side of the family was not to say "Is there a time when it would be convenient for me to visit you?" but "We're coming X through Y dates." In retrospect, my parents probably did have a problem with it ..... but they never really said no.
However, one family of cousins did finally end up getting banned. Of all the relatives, they were most prone to show up with very little notice, and acted with great inconsideration (scattering towels, never cleaning up, etc.) They ate as if it were going out of style and would eat ANYTHING in the fridge, without asking. They also expected my parents to put the three of us girls in one bedroom, leaving two bedrooms and a bathroom free for their exclusive use. (Yes, they actually said this - typically, adult visitors got a bedroom and visiting kids got "salted" in amongst the other children. They did not like that arrangement and said so.)
On BadCousins' last fateful visit, I was probably 14 and my youngest brother was around 7. They had three kids, the oldest of whom was 5 or 6, the youngest a toddler but still in diapers. I don't remember all the details, but I do remember:
*Because of the short notice, my mom (who was a WAHM) had commitments that took her out of the house, including some volunteer work with a summer program that my middle brother and sister were enrolled in. BadCousinMom took offense to this the first day and said so. My mom stayed home the next day ... and BadCousinMom promptly took off to go shopping, leaving Mom to watch her three (horribly behaved) children.
The next day Mom had to go volunteer again. BadCousinMom was very put out and snappy. And after an hour, she just left - that's right, went off and left her three small children with a 14-year-old. I think my youngest brother might have been there as well. I was experienced in watching children, but not this many, and the kids I usually babysat knew they were required to obey me. No such thing with these kids.

Things got so out of control that I called my paternal grandmother (who lived about 15 minutes away) to come help. When my mom came home (she got home before BadCousinMom) she went
ballistic. And when BadCousinMom got home, it was made
absolutely clear: They were never to darken our door again. Mom could handle the ongoing abuse of her hospitality, but not such a flagrant safety violation.