Learning to Take Turns
Sharing is hard for a long time, and it is learned slowly through practice.
Waiting for a turn asks a lot of a young child, and finding it hard is not rudeness. It takes practice and it takes years.
Games with an obvious rhythm help: rolling a ball back and forth, taking turns to stack a block, passing a toy round in a circle. Name what is happening — "my turn, now your turn" — so the pattern has words attached to it.
When two children want the same thing, describing what you see often works better than deciding for them: "you both want the red one." Give them a moment to find their own way through before you step in.
Cousins, neighbours and a park in the evening give more practice than any activity you could plan.