Chiron is a half-brother to Hestia, Hades, Poseidon, Hera, and Demeter. He is the son of Cronus and Philyra, an Oceanid. But, when was this consummation made? Before, after or during Cronus and Rhea were parenting the next generation of gods?
Younger, but not by much.
Chiron was conceived when Zeus was still a baby, and while Cronus was hunting his youngest son by Rhea. Cronus and Rhea were still living together as husband and wife during Cronus's clandestine union with Philyra. (Some interpretations I have read have it such that it was against Philyra's will.)
There was an island in the Black Sea which Apollonius Rhodius' Argonautica tells us was named Philyra, after this Oceanid. During their voyage the Argonauts sailed to this island,
where Cronus, son of Uranus, during the time in Olympus that he reigned over the Titans, and Zeus was yet being nurtured in a Cretan cave by the Curetes of Ida, lay beside Philyra, when he had deceived Rhea; and the goddess found them in the midst of their dalliance; and Cronus leaped up from the couch with a rush in the form of a stallion with flowing mane, but Oceanus's daughter, Philyra, in shame left the spot and those haunts, and came to the long Pelasgian ridges, where by her union with the transfigured deity she brought forth huge Cheiron [Chiron], half like a horse, half like a god.
Book 2, Lines 1231-1241
The Roman writer Hyginus corroborates Apollonius' account with his own:
As Saturnus [= Cronus] was combing the earth looking for Jupiter [= Zeus], he came to Thrace, turned himself into a horse, and lay with Philyra daughter of Oceanus. By him she bore Chiron the Centaur, who is said to have invented the art of healing. After Philyra saw that she had borne a strange species, never before seen, she asked Jupiter to change her into some other form. She was transformed into the philyra tree, that is, a linden tree.
In Greek mythology, however, Philyra lived with her son Chiron in his cave on Mt Pelion in Thessaly, together with Chiron's wife Chariclo, who was also one of her sisters, an Oceanid. According to Pindar's Pythian Ode, Chiron's daughters dwelt in the same residence as well.