SIL and BIL and their 3 children. When invited to stay over Christmas never commit. We assume they aren't coming then (usually with less than a weeks notice, but once with less that 48 hours notice) get in touch to confirm the plans (that we never had in the first place because they wouldn't commit). They will come for 2 nights. Never bring any of the bedding or towels we ask them to so we have to hunting through the attic for spare pillows etc. (And they inevitably complain they can't sleep with one pillow each. I don't know where they think we keep 10 spare pillows and 4 spare duvets all year).
One teen must have his own room, but insists it is the spare room with the double bed (further reducing bed space available). Then our three children end up in one room with two cousins. It is insanely hot with all 5 children in one room so one of their children moves to our office complaining about our air mattress and how much better their camp beds are (that they never fucking bring with them!).
They always say "you must let us bring something for a meal" so we say "great, bring lunch for one of the days- meat, cheese and some salad maybe" and they bring one loaf of bread, a small pack of cheese and some value tomatoes. This isn't enough for even their family, so on top of catering for 5 other meals during their stay (generously- roast Christmas meal, cooked breakfasts, another full dinner with options for their vegetarian etc) we have to rush to the shop to buy food for that other meal.
Their children gorge themselves. One year, one of then ate a whole giant babybel in one sitting causing our child to cry as they hadn't had any. (Luckily they were headed home after that meal as I can't imagine the aftermath was pretty).
The parents always have at least one full on screaming match. The mother usually tells the eldest scapegoat son that this is his fault (it isn't). Think standing in the street shouting and sobbing whilst we try to distract the children and have no idea what to say.
One of their children seems to manage to break some of our children's toys every time and will not listen to instruction. If challenged the parents say he is their youngest and so our children should let him have his way. But he is ages with our children. This child always gets his way. When he hits or bites his siblings and they eventually snap and hit back they are told off because he is the youngest. (He is 9).
Once they left the mother home whilst the others came, because as a mother of 3 children she really needed a break. So they came to stay with me. Also a mother of 3 children. Now looking after 6 of them for 2 nights so she could rest.
There is sooo much more (leaving dog shit all over the garden for example despite repeatedly asking them to pick it up). (And BIL licking his fingers then picking at food in the serving dish) (and BIL taking his socks off and picking his feet on the sofa in the evening 🤮)(and none of them close the toilet seat when they flush)
😡 I must stop.
And no I don't know why we do it, but I will say it means that DH is always very sweetly accommodating of my families crazy so it seems only fair.