Alright.
So we have Zeus turning into a Swan and seducing Leda, who was the Queen of Sparta, married to King Tyndareus at the time. She had 4 Children in 2 eggs. Castor & Polydeuces who would later become the constellation Gemini were her boys. Helen of Troy and Clytemnestra were the daughters.
It occurs sometime between 1300 BC and 1100 BC. It happened so far back in time and the Battle of Troy was the last great Mythological thing that happened and is also the beginning of the Historical text in Western Civilization, so it's mired in Myth and Fact and that's how they told their stories back then.
There aren't many texts to compare it with, but the Hittites and the Egyptians have some texts that talk about it.
Well, here's the rest of the story about the Twins, who died before a dozen years before the Battle of Troy:
Twins, Castor and Polydeuces ; were the most famous Spartan heroes. Some recorded them both as sons of Tyndareüs (Tyndareus) and Leda, daughter of Thestius, while others say that they were sons of Zeus, thus they were called Dioscuri. But most writers say that Castor with his sister Clytemnestra were Tyndareüs' children, while Polydeuces and Helen were Zeus', by Leda. Zeus visited and seduced Leda in the form of swan. So Castor was mortal, while his twin was immortal. The mother of Helen was quite often said to be Nemesis, the goddess of retribution, whom Zeus had seduced in the form of a swan. According to the Homeric Hymna, to the Dioscuri, Leda bore the twins on the peak of Mount Taygetus, but it doesn't state that their sister, Helen was born there too.
Castor became renowned as a horseman, and was given a title of Tamer of Horses. While Polydeuces was renown for his skills in boxing. The Dioscuri were identified riding a pair of white horses.
Castor had trained the youth Heracles in fencing. They sailed with the Argonauts, where Polydeuces killed Amycus, king of the Bebrycians, when challenging the crew in boxing match. They also joined with several members of the Argonauts in the Calydonian boar hunt. The Dioscuri and Jason also helped Peleus in capturing Iolcus from Acastus.
When Theseus and Peirithoüs (Peirithous) abducted their sister Helen, the Dioscuri captured Athens and returning Helen to Sparta, with Theseus' mother held as captive. Theseus also lost the throne to another Athenian named Menestheus whom the Dioscuri install as king of Athens. (Menestheus was later a suitor of Helen. He led fifty ships to Troy.)
Their main rival and enemies were their cousins: Idas and Lynceus, sons of Aphareus. When Idas and Lynceus were going to married their cousins, Phoebe and Hilaera, daughters of Leucippus, the Dioscuri abducted them and married the girls themselves. Hilaera bore Anogon to Castor, while Phoebe bore Mnesilus to Polydeuces.
When they raided some cattle with Idas and Lynceus, they decided on the contest of who get all of the cattle. Idas and Lynceus won the contest and drove their prize back to Messene. Castor and Polydeuces decided to take the cattle back in a raid. Lynceus warned Idas of their approach and ambushed the Spartan twins. In the fighting that followed Polydeuces killed Lynceus, but was felled by rock hurled by Idas. Zeus protecting his son killed Idas with a thunderbolt.
Polydeuces was grief-stricken by death of Castor at Idas' hand. The immortal Polydeuces wanted to die with his brother. Taking pity on his mourning son, Zeus decided that the twins would share their immortality: they would spend their days living alternately both (or separately) in Olympus and the Underworld.
Zeus placed them as constellation, Gemini, in the heaven. They became gods in Sparta, where they were patrons of warlike youths and sailors in the stormy sea.
According to Homer in the Iliad, Helen wondered where her brothers were, because she did not see them among the Greeks fighting in the war, not realising that they had already died. This suggested that her brothers, Castor and Polydeuces, had died after she ran off with Paris to Troy.