[Grammar] older versus oldest

[Albert McKinney]

When talking about the oldest person out of a group of three or more people, it is obviously correct to refer to that person as the oldest. But if the group has only two persons, some people object to calling that person the oldest, and insist that the correct usage is to call that person the older. Is there any justification for this distinction?

 

[rlfwood]

from Adjectives

"Adjectives can express degrees of modification:

Gladys is a rich woman, but Josie is richer than Gladys, and Sadie is the richest woman in town.

The degrees of comparison are known as the positive, the comparative, and the superlative. (Actually, only the comparative and superlative show degrees.) We use the comparative for comparing two things and the superlative for comparing three or more things. Notice that the word than frequently accompanies the comparative and the word the precedes the superlative. The inflected suffixes -er and -est suffice to form most comparatives and superlatives, although we need -ier and -iest when a two-syllable adjective ends in y (happier and happiest); otherwise we use more and most when an adjective has more than one syllable."

 

[Linguist__]

(Not a teacher)

I think you could say either.

"A is the older of the two brothers."
"A is the oldest of the two brothers."

To me, both are correct, although, 'eldest' is more commonly used for siblings (/groups of people?).