These two goddesses are not half sisters. Zeus was married to Metis before he was married to Hera. He swallowed Athena before she was born and later he had a pain in his head. Hephaestus used his magic ax on Zeus's head. Athena was born fully grown wearing armor from the head of Zeus.
Iris is the daughter from the titan Thaumas and the cloud nymph Electra.
Persephone was Athena's half-sister. The both had Zeus as father. Persephone sometimes is depicted as a child of Demeter alone. They were also cousins, since Demeter and Zeus were also brother and sister.
ares Hermes aphrodite Artemis hestia
During the big war with the giants she was married to hestaphus
She did not have any brothers or sisters. She was the only child of Metis and Zeus. Legend says that she might have had a brother that could have overthrown Zeus. However, all the children of Zeus are her half-siblings.
The half-sister of your granddaughter's half-sister is your granddaughter, or is not related to you.
Her half-brother was Mars or Ares (same person) *Total Athena Genius* (TAG)
Iris served the gods of Olympus. Her sister served the Titans.
The Iris Choir
You grandmother's half-sister is your half-grandaunt.
You have no blood line relation, You don't share DNA whatsoever. In this scenario putting an example; you and your half-sister share, let's say, the same father but not the same mother and your half-sister's half-sister share the same mother but not the same father. There is no shared DNA between you and your half-sister's half-sister.
King Tuts wife was his half-sister.
Roald Dahl called his sister Ellen his "Ancient Half Sister."