Answer:
Sebastian meets Feste, who mistakes him for Viola/Cesario.
Olivia mistakenly thinks Sebastian is Cesario.
Feste, disguised as Sir Topas, ridicules Malvolio in the dark cell.
Sebastian thinks that he is suffering from memory loss or has gone mad.
Olivia gets a priest and marries Sebastian, still thinking he is Cesario.
Explanation:
Sequence