Percy is related to both Medea and the Chimera
Medea is the daughter of Helios. Helios is the son of Hyperion. Hyperion is Kronos's brother. Kronos fathered Poseidon, who fathered Percy.
Some people have speculated that Echidna is Chrysaor's daughter. Chrysaor was Poseidon's son. Echidna gave birth to the Chimera.
Others say she (Echidna) is most likely the daughter of Phorcys and Keto. Phorcys is Nereus's brother. Nereus is the father of Amphitrite, who is married to Poseidon, who fathered Percy. And again, the Chimera is Echidna's child (with Typhon/Typhoeus)
Of course, nearly everyone in mythology is related, due to the incest...