Castor and Pollux

Castor and Pollux, are the sons of Leda and yet the fatherhood is unsure. Jupiter had beguiled her as a swan and some say that Castor caught the mortal gene and Pollux (Polydeuces) the imortal. Others say that on the same night she slept with Tyndareus, her husband. Four children were born, that's agreed. Daughters Clytemnestra and Helen. Some say that Castor and Clytemnestra are Tyndareus children and are mortal, whilst Helen and Pollux are Zeus's and imortal. Given that Castor died then maybe he was Tyndareus's son.

When Theseus and his friend Pirithous had carried off Helen from
Sparta, the youthful heroes Castor and Pollux, with their
followers, hasted to her rescue. Theseus was absent from Attica,
and the brothers were successful in recovering their sister.

Castor was famous for taming and managing horses, and Pollux for
skill in boxing. They were united by the warmest affection, and
inseparable in all their enterprises. They accompanied the
Argonautic expedition. During the voyage a storm arose, and
Orpheus prayed to the Samothracian gods, and played on his harp,
whereupon the storm ceased and stars appeared on the heads of the
brothers. From this incident, Castor and Pollux came afterwards
to be considered the patron deities of seamen and voyagers (One
of the ships in which St. Paul sailed was named the Castor and
Pollux. See Acts xxviii.II.), and the lambent flames, which in
certain sates of the atmosphere play round the sails and masts of
vessels, were called by their names.

After the Argonautic expedition, we find Castor and Pollux
engaged in a war with Idas and Lynceus. Castor was slain, and
Pollux, inconsolable for the loss of his brother, besought
Jupiter to be permitted to give his own life as a ransom for him.
Jupiter so far consented as to allow the two brothers to enjoy
the boon of life alternately, passing one day under the earth and
the next in the heavenly abodes. According to another form of
the story, Jupiter rewarded the attachment of the brothers by
placing them among the stars as Gemini, the Twins.

They received divine honors under the name of Dioscuri (sons of
Jove). They were believed to have appeared occasionally in later
times, taking part with one side or the other, in hard-fought
fields, and were said on such occasions to be mounted on
magnificent white steeds. Thus, in the early history of Rome,
they are said to have assisted the Romans at the battle of Lake
Regillus, and after the victory a temple was erected in their
honor on the spot where they appeared.

MaCAULEY: Lays of Ancient Rome

"So like they were, no mortal
Might one from other know;
White as snow their armor was,
Their steeds were white as snow.
Never on earthly anvil
Did such rare armor gleam,
And never did such gallant steeds
Drink of an earthly stream.
. . . . . . . . .

"Back comes the chief in triumph
Who in the hour of fight
Hath seen the great Twin Brethren
In harness on his right.
Safe comes the ship to haven
Through billows and through gales,
If once the great Twin Brethren
Sit shining on the sails."

In the poem of Atalanta in Calydon Mr. Swinburne thus describes
the little Helen and Clytemnestra, the sisters of Castor and
Pollux:

MELEAGER

"Even such I saw their sisters, one swan white,
The little Helen, and less fair than she,
Fair Clytemnestra, grave as pasturing fawns,
Who feed and fear the arrow; but at whiles,
As one smitten with love or wrung with joy,
She laughs and lightens with her eyes, and then
Weeps; whereat Helen, having laughed, weeps too,
And the other chides her, and she being chid speaks naught,
But cheeks and lips and eyelids kisses her,
Laughing; so fare they, as in their blameless bud,
And full of unblown life, the blood of gods."

ALTHEA

"Sweet days before them, and good loves and lords,
And tender and temperate honors of the hearth;
Peace, and a perfect life and blameless bed"