1 definition by OWO777

When your parents choose one sibling and is always on their side for everything. They can never do anything wrong. Think of it as sexism and racism
Mom: Tommy you are such a good boy

Me: What! I'm good too!
Mom:...
Me: This is complete siblingnisum
by OWO777 November 10, 2021
Get the Siblingnisum mug.