article "a"

[Pooh]

"Besides being brother and sister, they are also good friends."
In this sentense, how come there is no article "a" in fron of "brother"?

Thanks in advance.

[Mordant]

When we are expressing familial relationships between people, the article is usually omitted.

Sandra and Alex are mother and son.
It was a great experience between father and son.

Adam is a brother and a son. - No relationship to anyone else is expressed.

Besides being a brother and a sister, they are also good friends.

That sentence sounds awkward if we are expressing their relationship to each other. It is also how someone might explain that one happens to be someone else's brother and the other someone else's sister.

[Pooh]

He has been elected mayor of the city.

How come this sentence has no "the" in front of mayor?

Thank you as always in advance!!

[Mordant]

Oftentimes the article is omitted when someone enters a specific position or role, especially when they do so as a result of an action.

The board made him president of the company.
He was voted treasurer Wednesday.
She became council president.
I was appointed company chairman.
He was installed as congressman.

She is mayor. - I have a specific mayoralty in mind.
She is a mayor. - This is what she does, and the location is irrelevant.

[Pooh]

[Mordant]

It would be correct to retain "the" as well. This is optional.

He was elected the president of the United States.
He was elected president of the United States.

These sentences are equivalent.

If you want to refer to a general position or role, however, "a" is required.

She is an attorney. - No specific job in mind, just occupation

This idea would not be correctly conveyed without the indefinite article.

[Pooh]

Thank you very much!

I tried to delete the other thread for "the" but I failed.
Hope somebody can do it for me.
Sorry for the troublesome matter.