My sister has four children: an 6-year-old, a 5-year-old and 2-year-old twins.
The older two are easy visitors. They tend to sit quietly, chat a bit and spend a lot of time on their screens. The twins, however, are typical two-year-olds. They run around, make noise, have the occasional tantrum and create a bit of mess.
DM is very house-proud and likes everything neat, tidy and pristine.
She has become increasingly irritated by the twins whenever they visit. She complains that they don't sit nicely, don't behave properly and are too noisy. Personally, I think her expectations of two-year-olds are unrealistic.
The latest development is that she has told DSis that the twins are no longer welcome at her house. She says DSis can bring the older two, but not the younger ones.
The problem is that DSis's husband works long hours and isn't always available to stay home with the twins. So in practice, this means DSis often can't visit at all, or has to choose between seeing DM and staying with her younger children.
DM says it's her house and she's entitled to decide who comes into it. DSis feels hurt that two of her children have effectively been singled out and excluded.