Moe has two brothers and a sister, as his remaining family after his Mother passed away from severe Leukemia and his Father sacrificed himself to save Moe and Starfy during Starfy 3.
The green one is the elder sibling. His name is Do.
The purple one with the bow is second eldest. Her name is Re.
The red one with the binkie is the youngest. His name is Mi.
Put their names together, and you get DoReMi!