If you are at home among your family, it's up to your family where the line is (especially for young kids.) Kids need screaming time *sometimes* and if "outside" or "in your own room" aren't possible, well, it's up to parents to decide on the compromise. (Honestly, I need screaming time sometimes!) And at home, handwashing and toilet flushing really does fall under let me parent my own kids - if I was visiting someone else's house, I might think it was gross that they didn't all wash their hands before lunch, but I wouldn't criticize their kids for, it either, and a lot of that stuff can be about choosing your battles.

If they're "at home" when they're at LW's house, there's discussions to be had and compromises to be made about to what extent they need the freedom to make their own choices in their own home, and at what point they need to agree to shared standards with the other people to live there and how to be consistent about them with the kids. Critically, they're discussions best had with the adults as a group, not by LW snapping at the kids.

However if LW's house isn't their home they should be always flushing toilets - with Mom checking if they don't - and being removed from the house when unwilling to stop screaming, no discussion needed.