Leda

... was also the mother of the twins Castor and Pollux, and of Clytemnestra (the wife of Agamemnon, King of Mycaenae, who commanded the united Greek forces in the Trojan War).

According to myth, Zeus appeared to Leda in the form of a swan, seducing (or, if you prefer, raping) and impregnating her. Leda produced four offspring from two eggs: Castor and Clytemnestra from one egg, and Helen and Polydeuces (Pollux) from the other. Therefore (says Wikipedia – I'm not sure how this follows), Castor and Clytemnestra were fathered by Leda's husband, the Spartan king Tyndareus, while Helen and Polydeuces were fathered by Zeus.