Closely related:
obelia
jellyfish
Not closely related (but still related):
coral
sea anemone
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.
damselflies
incest.
Animals
Organisms in the same genus are most closely related to one another according to Linnaeus's groupings.
Humans and chimpanzees are closely related organisms, sharing a common ancestor. Additionally, dogs and wolves are closely related, with domestic dogs descending from wolves through selective breeding.
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.
Family and kingdom
Same genus or closely related species.
Clasifiaction
Thesus monkey is the most closely related to Humans because it has the least number of differences in cytochrome c.