This source names Apollo directly: "and the Titaness Leto, twin sister of Apollo."
Source The Homeric HymnsAnonymous; ascribed in antiquity to Homer
This source names Artemis directly: "Ares, Aphrodite, Apollo, Athena, Artemis, Hermes, Maia, the Moirai, Nereus, Doris and Okeanos, plus the separate scene of Hephaistos riding a mule back to Olympus."
Source Florence 4209, the Francois Vase (Perseus Project Art and Archaeology Artifact Browser)
This source names Artemis directly: "Ares, Aphrodite, Apollo, Athena, Artemis and Hermes are named among the deities present, alongside the sea deities Amphitrite, Nereus, Doris and Okeanos and other attendant figures."
Source Florence 4209, the Francois Vase (Perseus Project Art and Archaeology Artifact Browser)
Leto, Deities This source names Leto directly: "the daughter of Zeus and the Titaness Leto, twin sister of Apollo."
Source The Homeric HymnsAnonymous; ascribed in antiquity to Homer
This source names Artemis directly: "Hera, Poseidon, Athena, Dionysus and Artemis some five hundred years before Homer, which established that those gods were worshipped in the Bronze Age and not invented later."
Source The Decipherment of Linear BJohn Chadwick
This source names Artemis directly: "slaying of her children by Apollo and Artemis, punishment for her boast against Leto."
Source The Punishment of Niobe, accession 2020.278 (Metropolitan Museum of Art)
Zeus, Deities This source names Zeus directly: "tradition Artemis is the daughter of Zeus and the Titaness Leto, twin sister of Apollo."
Source The Homeric HymnsAnonymous; ascribed in antiquity to Homer