Without the comma, "dearest" is a modifier to Angelica, it's a term of endearment, but not uncommon as a familial term, particularly in that era. The comma separates "Angelica", and makes "my dearest" it's own form of address. A man calling a woman "my dearest" implies a connection that's more romantic that sisterly. It makes it less a typical salutation and more an expression of their unrequited attraction. | Hamilton
