So, we've met Viola, who is in mourning for her brother who she believes was lost at sea, and who is now masquerading as a boy who just happens to look exactly like the aforementioned brother.
Let's meet the brother!
Boy/girl twins are never identical, but DON'T LET THIS FACT DISRUPT YOUR SUSPENSION OF DISBELIEF. Just go on accepting that "Cesario" looks exactly like Sebastian. It'll make the subsequent plot hijinks much easier to digest.
Also, someone give Antonio a hug. He needs a hug.