Not sure why you have titled this "MIL drops kids off Boxing Day".
The text describes it is your sister-in-law who claimed to be ill and left HER children at your home. Not your Mother-in-law. Why is MIL getting the blame here?
You've also said "My DH’s SIL" is a single mum and her parents...., granted her parents were there....
So isn't this your husband's sister, not his sister-in-law?
Anyway, if there were 10 kids in total, including yours and some from your side of the family, there were a lot of other adults there besides you. Your husband, his parents and possibly some relatives from your side of the family. Why are all these people sitting back and ignoring the 10 children? Your problem lies more with them.
And if your own children are allowed to eat upstairs, it's a little unfair to pick up on the visiting children doing the same thing. You can't have different "rules" for the cousins you like better and your own children than for your nieces and nephews you don't prefer.
At a family event, the entire family should be the "Village". Yes, your sister-in-law may or may not be genuinely unwell, but it's all the other adults who could have pitched in with food, games and watching the kids, rather than putting their feet up, leaving you to juggle everything, who have let you down.
Focus some of your attention and disquiet there.