According to my lecture notes, adding a red child to a red node with a black sibling in a red-black tree, is equivalent to changing a 3-node into a 4-node in a 2-3-4 tree. I've built several red-black trees with different values to try to get to this situation, but I never seem to get to the situation where I have a red node with a black sibling, and where the value to be inserted will be inserted below the red node. I'm starting to ask myself if this situation could ever happen in a red-black tree?
Red-black tree. Adding a red child to a red node with a black sibling
data structures