LW and any host is free to have rules about cleaning up after oneself in their home - you can just not invite people, disinvite them or ask them to leave, and if the poor behavior is from children it's completely reasonable to remind them to flush the toilet and wash their hands and to ask them to keep their voices down, for example. All of these are requests that the child be courteous to the host and the people around them, and the mother is mistaken in categorizing them as "parenting her children". Yes, it's her resonsibility to tell them these things, but that's because the parent is responsible for her children's behavior. LW would be within their rights to explain to HER that people are not welcome in their house who don't flush the toilet and wash their hands afterwards, or not welcome to eat in their house without washing their hands first, or not welcome to make the room too uncomfortable for other people to be in, and then expect her to enforce this on the children instead. Or to address themselves to her instead of to the children in the moment of each infraction ("Alice, Billy hasn't washed his hands and he isn't welcome at the table until he has." "Alice, I'm afraid the children are being too loud again - if they can't calm down, you could ask them to go outside perhaps.") Perhaps the mother would prefer this, but I think it sounds like she actually just wants the host to let her do whatever she wants in their house and... uh... that isn't reasonable at all. And not how going to other people's houses works, even if the people in question are your in-laws.