It does seem to me that the grandparents are willing to compromise - they're willing to compromise by having the kids stay in a hotel when they visit. It's not ideal, but sometimes people have different accessibility needs that conflict and, sorry, but on two different metrics I agree that this nephew gets priority - first, because he's there every week, and second because his disability looks like it affects him a lot more than the kids' allergy. It sucks, and it sucks all around, but he's not the kid I'm going to inconvenience.