On Act II we see Viola asking where she currently was and then she finds out that her twin brother was in another country. The dialogue below shows how nobody there could control what happened (or would happen) to her brother, leaving that for his own fate to decide:
VIOLA
And what should I do in Illyria?
My brother he is in Elysium.
Perchance he is not drown'd: what think you, sailors?
Captain
It is perchance that you yourself were saved.
VIOLA
O my poor brother! and so perchance may he be.
After the shipwreck, Viola ended up in a country named Illyria, while her brother was in a country named Elysium. They didn’t control what led them to this separation (the shipwreck itself), they couldn’t control who survived (and who didn’t) nor their location after the accident. That said, we can support Malvolio’s idea of the character’s fates depending more on external circumstances than their own traits and choices.
