Comma usage (ex. His sister, Anne, was not feeling well.) [duplicate]
The traditional explanation for when to use commas around Anne is as follows:
If the unidentified he in the sentence has only one sister (Anne), then the word Anne is functioning as an appositive, and you would set it off with commas:
His sister, Anne, was not feeling well.
But if the he in the sentence has two or more sisters, the word Anne is identifying which one was not feeling well and so would not be set in commas:
His sister Anne was not feeling well.
Here is a discussion of how to punctuate appositives, from Words Into Type, third edition (1974):
Appositives. Set off words in appositives by commas
[Relevant example:] He was replaced by a German leader, Odoacer, and thus a ruler from the barbaric tribes was recognized in Rome.
...
Restrictive appositives. A restrictive appositive is one used to distinguish its principal from other persons or things of the same name, group, or class. Such an appositive, which could not be omitted without robbing the sentence of meaning, should not be set off.
[Relevant examples:] the poet Longfellow; his brother Will; my friend Pat
So in your example, if Anne is his only sister, Anne is a regular appositive (and requires commas), but if Anne is one of his two or more sisters, Anne is a restrictive appositive (and should be left unpunctuated).
If the example is direct address (hint: his sister's name is not necessarily Anne), the commas are required.
If the sentence is not direct address, whether or not the commas are required may depend on the stylistic context, among other things. For example, The Chicago Manual of Style, 14th Edition, says this: "Unless it is restrictive (see 5.50), a word...that is in apposition to a noun is usually set off by commas...." (5.49, italics mine). Then, "If the appositive has a restrictive function, it is not set off by commas...." (5.50).
The Associated Press Stylebook (2007, p. 326) takes what at first appears to be a more didactic (more prescriptive than the comparatively more descriptive Chicago Manual) approach: "A nonessential phrase must be set off by by commas. An essential phrase must not be set off by commas."
I say 'at first' because, when you examine what's intended by "essential" and "nonessential" (pp. 87-8), you find this:
These terms are used in this book instead of restrictive phrase and nonrestrictive phrase to convey the distinction between the two in a more easily remembered manner.
The underlying concept is the one that also applies to clauses:
An essential phrase is a word or group of words critical to the reader's understanding of what the author had in mind.
A nonessential phrase provides more information about something. Although the information may be helpful to the reader's comprehension, the reader would not be misled if the information were not there.
So, in the example you give, whether the appositive phrase is essential or nonessential depends on your sense (a sense sponsored by the stylistic and semantic context) of whether or not the information is essential or nonessential.