Leda's children

Leda and the Swan - Francesco Melzi, from a lost work of Leonardo da Vinci

The Greek fascination with symmetrical pairs is latent in this myth of Leda giving birth to two eggs, each holding twins. One egg held the Dioscuri, Castor and Polydeuces. As with the tale of Theseus, the parentage of the Dioscuri was uncertain. It seems Castor was actually the child of Tyndareus, and therefore mortal. Polydeuces was the immortal son of Zeus.

So much for the oversimplified phrase, "point of origin."

The same goes for the other egg, which contained Helen and Clytaemnestra -- Helen, of Zeus, and Clytaemnestra, of Tyndareus. These "twin" sisters married the brothers, Menelaus and Agamemnon, sons of Atreus and Aerope. 

Aerope, daughter of Catreus and therefore granddaughter of Minos, contributed to the horrific division between Atreus and Thyestes. Indeed, traced in detail, the trajectories of Catreus and his descendants had fateful impacts not only on the Pelopids, but later down the line on Menelaos, Agamemnon and Cassandra.