Who were sebastian and viola

Viola is a young woman who is shipwrecked, having lost her twin brother Sebastian, and lands in Illyria quite by accident; she resolves to make the best of her situation, and be taken into Orsino's service, as a young eunuch named Cesario.

Sebastian is Viola's non-identical twin brother, who nevertheless bears a great resemblance to her. Viola tries to imitate her brother in her disguise as Cesario, as the two are mistaken for each other in many scenes.