Closely related:
obelia
jellyfish
Not closely related (but still related):
coral
sea anemone
Jellyfish and Corals
Organisms that are closely related to rabbits include; horses and turtles
You can tell how closely related organisms are by determining the lowest classification that they share. For example, organisms that share the same genus will be more closely related than organisms that share the same family.
My weiner
The organism most closely related to the snake is the lizard.
incest.
Animals
damselflies
A clade is a group that includes an organism and all its descendants that share a common ancestor. It represents a branch on the evolutionary tree of life.
Fire corals are colonial marine organisms that look rather like real coral. Technically they are not corals, since they are more closely related to Hydra and other hydrozoans.
Mergus serrator and Mergus merganser.
Family and kingdom
The group of organisms where members are more closely associated is species. Genus is the next one group where members are closely related.