Scenario: My sister has four children, all in their 20s. DC #3 has a birthday next week and has asked for the family to go out to dinner. DC #3 has said she doesn't want to invite DC #1. My sister (the mother) is paying for the dinner. Is it reasonable to exclude one sibling out of four knowing the sibling will be hurt terribly? My sister isn't sure if she should deny the request for a birthday dinner or not just to avoid the situation. It's their birthday but the mother doesn't like being part of the exclusion, which she is if she throws the dinner and has to not invite someone, knowing their older child will probably ask if there is a dinner. It's over a basic philosophical disagreement between them, not something serious that would make it understandable. How would you handle it?