This is something that I've randomly put together. It's all the relationships between demigods, gods and other people.
Chiron's dad is Kronos- that makes Chiron half-brother to Zeus, Poseidon, Hades, Hestia, Demeter and possibly Hera.
Kronos's parents are Gaea and Ouranos.
Kronos is the grandfather to all the children of Zeus, Poseidon, Hades and Demeter.
Poseidon's demigod children- (OC included) Percy Jackson and Louisa Smith-Jackson and Tyson, although he's not really a demigod.
Hades' demigod children are Bianca and Nico di Angelo and Hazel Levesque, who's Roman, so that makes her Pluto's demigod child.
Zeus' demigod children are Thalia and Jason Grace.
Uncles to the Big Three kids are Hades, Zeus, Poseidon and Chiron.
Aunts to the Big Three kids are Hestia and Demeter.
Zeus' godly children include Dionysus (who was originally a demigod but his mortal mom was vaporised because she saw Zeus' true godly form. Dionysus was stitched in Zeus' thigh until he was ready to be born and that's why he's immortal), Apollo and Artemis (twins- mother is Leto), Athena (who has no mom 'cos she was born from Zeus' cracked skull, fully grown and in battle armour), Ares, Hephaestus (who was thrown off of Olympus as a baby for being too ugly- either Hera or Zeus is to blame) and Hermes.
Aphrodite and Hephaestus married, which makes their demigod children step-siblings.
Dionysus, Apollo, Artemis, Athena, Ares, Hephaestus and Hermes are all cousins to Percy, Louisa, Bianca, Nico and Hazel. The gods are half-siblings to Jason and Thalia.
Triton is the half-brother of Percy, Louisa and Tyson. All of them are half siblings to the original Pegasus and Chrysaor- both of who were born from the bleeding stump of Medusa's neck, sired by Poseidon, after she was beheaded.
Hades' nephews and nieces are Percy, Louisa, Thalia and Jason.
Zeus' nephews and nieces are Percy, Louisa, Bianca, Nico and Hazel.
Poseidon's nephews and nieces are Thalia, Jason, Bianca, Nico and Hazel.
The Big Three kids are grandchildren of Kronos and great-grandchildren of Gaea and Ouranos.
Amphitrite is the stepmom to Percy and Louisa.
Hera is the sister and wife of Zeus, but that makes her the sister of Poseidon, Hades and Chiron – some say she is the sister as well. This makes Hera another aunt to the Big Three kids.
Chiron had a wife (according to Wikipedia) so, by marriage, she is aunt to the Big Three kids as well.
All the children of Dionysus, Athena, Apollo, Hephaestus, Ares and Hermes are all cousins.
Hercules, son of Zeus (I think) is half-brother to Thalia and Jason and cousin to Percy, Louisa, Bianca, Nico and Hazel.
If you can think of any more, please tell me and I'll post them up and say who they're by.