I am a married SAHM with 2 kids, ages 7 and 9. My sister is a married working mom with 2 kids, ages 7 and 10. My parents are retired and frequently invite my sister's children to spend the night on school holidays and will take my nephews to play at the park, etc. on their days off of school. My parents have NEVER invited my kids to spend the night or offered to take them anywhere.
I suspect that my parents extend these invitations to my sister's kids to help her out with childcare. My kids are hurt that Grandpa takes their cousins places, but has never invited them. For the record, both sets of children are well-behaved.