Explain the circumstances leading to Sebastian's words: I am sorry, madam, I have hurt your kinsman

The line comes from act 5, scene 1, of Twelfth Night. An injured Sir Andrew enters, calling for a doctor. He claims that he and Sir Toby Belch have just been set upon by Cesario. As we might have expected by now, this is a case of mistaken identity, for it wasn't Cesario—a.k.a Viola—but her long-lost twin brother Sebastian who attacked the two men. After Sir Andrew and Sir Toby depart, Sebastian arrives on the scene and immediately apologizes to Olivia for his actions, not least because Sir Toby is Olivia's cousin. But he had no choice, he claims; he only acted in self-defense:

I am sorry, madam, I have hurt your kinsman,
But, had it been the brother of my blood,
I must have done no less with wit and safety.

Sebastian's telling the truth. The hapless duo set upon him in the previous act, wrongly believing him to be Cesario. Sir Andrew Aguecheek has always been incredibly jealous of Cesario, seeing "him" as a rival for Olivia's affections. Egged on by his drunken friend Sir Toby, Sir Andrew engages in a comical duel with whom he believes to be Cesario, but who in actual fact is Sebastian.