As stated above "das" is the common demonstrative pronoun - independent of gender and number. You can thus say
Das sind meine Brüder.
And it would be the generic choice. But "der/die/das" is a demonstrative pronoun as well. So what you are seeing is technically not the definite article, but a pronoun that is identical to it! As a demonstrative pronoun alone "der/die/das" has 10 sub uses in my dictionary.
But I believe what you are interested in here is the contrasting use of the demonstrative "der".
I try to make an example where both "das" and "der" are used idiomatically, the first neutrally and the second for contrast:
Wer ist der Mann mit der Sonnenbrille?
Das ist mein Freund!
Und was ist mit Martin?
Der hat mich verlassen!
Who is the man with the sun glasses?
That's my boyfriend!
And what about Martin?
He (contrasting) has left me!
More examples of "der/die/das" as a contrasting demonstrative pronoun include:
Welcher ist dein Vater? / Which one is your father?
Der mit der Glatze... / The one with the bald head.