Orsino calls Cesario to follow him, Olivia calls him to follow her and in doing so she accidentally on purpose slips and call him her husband. The priest, who has just entered, confirms that the two have been properly wedded not more than two hours ago. Viola is flummoxed, although honestly she should have realized by now it’s her twin brother. Orsino is very angry and like “get off my face, I don’t want to see any of you anymore”. I know it’s nitpicky of me and you should not look for the narrative logic in Shakespeare’s comedies, but since Illyria seems to practice some form of Christianity, I would like to to point out that both in the Catholic marriage service, in the Book of Common Prayer and probably in every Christian wedding service under the sun during the wedding vows the participants say their own names and the name of the person they are getting married to. Didn’t it seem strange to Sebastian that Olivia called him Cesario? Or didn’t Olivia hear the man she took for Cesario to call himself Sebastian? And if Sebastian for some inscrutable reason just decided to play along and call himself a false name, is the marriage even valid?
Let’s step over this huge narrative hole and go on. Sir Andrew appears, calling loudly for a surgeon, since he and Sir Toby have just been beaten up and wounded by Cesario. Olivia sends them home to bed. Then the cause of this whole misunderstanding, that is Sebastian, himself steps out on stage. Again, I find it a bit strange that he fails to notice the sister he claims to love so much , but perhaps it could be remedied in the production by hiding Viola in the crowd , and there are quite a lot of people on stage at the moment. Instead he first addresses Olivia, apologizing to her for beating up her relative, but he did it in defence of his own body and honour. Then he notices Antonio and is overjoyed at finding him at last. Meantime, Orsino notices the uncanny similarity between the two and is bemused by it.