Who is the grandmother to Apollo and Artemis and is the daughter to Uranus and Gaea?

Leto, their mother, her parents (their grandparents) KoiOS and Phoebe.

Asteria, sister of Leto.

Zeus, their father, his parents (their grandparents) Rhea and Cronus.

Hades, Poseidon, Hera, Hestia, Demeter, his siblings.

Any son or daughter of Zeus would be their half sibling including:

Ares

Hephaestus

Hermes

Athena

Dionysus